CZ's and Customer satisfaction

  Try  "gold  buyers":  often these  store operators  take  out the  stones and have   jars full  laying  around ( although  not  in a 6.5mm size as that  will run - even  from a "store" that  knows  zero  about  jewelry  but has the capitol to begin buying and  selling  metals ( which  pretty much  sums  up 90% of the  proliferation of  "gold  buyers" out there) in the  thousands- for  a 6.5mm stone.To buy from a wholesaler to the trade you will probably need to  have  a budget  of  7-8 thousand for a  fair stone..not good, not great just big..  even  if they  offer you  what's  called ' rose  cut ' ,included  and  a take  out! Even these resellers that  have no real  experience  and  less knowledge  in  most cases realize the  size you  are  looking for is not  usual from the  hundreds  of  "bridal sets" that  have potentially crossed  their store's threshold and the "take outs' came from .
   In  your  situation I  would  try and explain to  your  client that  a white  sapphire ,topaz or  zircon,  if a natural  stone  is preferred or insisted  upon, would be  available  in  that budget  range...some stones have higher  refractive indices  than  others  so  you would  have to  see the actual stone you are  buying  rather than  trying to  find it  online without the  possibility of  inspection before  payment...a tricky proposition..best to have a dealer  you know or have  had  prior relationship with in  business or  engage a jeweler  you know  with a reliable supply chain to  help..                                          
   Otherwise top of the line  cubic zirconia ( stones that  are optically flawless and will give  plenty of  sparkle to  a ring)   are quite  a bit under the $500 budget .Really  high quality  CZ's don't  look  as cheap  as  lower quality stones.A lower  quality  CZ has  a visible to the naked  eye  spectrum at every angle, while  "diamond simulants" other than  Moissanites are stones  of the  same chemical composition  but  produced by a different  method than the fast growing CZ's sold in quantity  or Moissanites- a risky simulated  diamond as they are very  very brittle, and in  my  opinion  nowhere near worth the price for the end  product that varies  batch to  batch, with many recent  recalled lots due to  imperfections and  breakage, even though they are " guaranteed"). These are  called CVD 's  or  CVD process(ed) - carbon vapour deposition processed cubic zirconia.    
  This type  has fairly recently ( 1990's or  thereabout) become available in  white or colourless stones,and  until recently  were  available  only  up  to  about 1.5 carats- far  from the 6.5mm you seek ! ( the density of natural diamond material is  less than that of cz by any  process so  estimating isn't equal to  estimating the weight of a diamond of that size into carat weight).An example is one of the largest recently sold on a popular jewelry  e-commerce site: a 1.24-carat princess-cut diamond, J color, VS1 with very good cut graded by the International Gemological Institute (IGI) was priced at $3,494.82 as a loose stone! A Moisannite is  yet higher, with natural  diamond still higher for  the same grade, etc. certification.
  ..While  cheap  CZ's  are available  in any size and a range of  colours ( except the white and colourless varieties of the simulants not possible in  "regular" mass produced cz's) for  next to  nothing to the trade. For  instance  one of  my favourite Jeweler's tool,supplies  and equipment vendors   has 7 mm x 7 mm square cz's  on  sale  at the  moment for $1.56 USD each, compared to the regular  wholesale price of $2.60 each! )  large CVD cubic zirconia is about 20% less than natural  diamond.Also cz's produced just  after the  dawn of the science was  being explored ( around the 1930's- when crystals of cz large enough to be cut and  faceted were first produced at the turn of the 20th century) can also  run about half the price of natural diamond  material- provided the sellers of  such vintage "costume" jewelry  know  what they have- these stones are generally  found in high end antique dealer's shoppes or those  jewelers  that  actually know estate  jewelry and  have  extensive testing equipment to  back up their claims ( and  usually some GIA  or equivalent  certification displayed in the  store) !
 So why you  undertook making a mounting without the stone seems a bit  backwards, to me anyway, the reality of finding  what you may have "promised" your client is an entirely different task!  First I'm betting you are  a novice to jewelry making and  as such  what you do want to try and  find is a machine  cut stone- no matter  what it is made of, provided  your  calculations  were correct in  making the mounting  for a 6.5mm anything.hand  cut stones  vary  in girdle, and pavilion.So provided you have been  exacting  in  your model, or setting making until now, at this point  you are either locked into  that size, or can remove the setting from shank and melt it down , roll it out and rework it to a more suitable stone for the  budget or  buy another white stone altogether ( the sapphire, topaz, spinel, or even a silver tourmaline are some possibilities) with the  client's knowledge..If you signed  any contract- now is the time to admit  your knowledge of " the current  diamond market  is lacking " and refund their deposit(s) or otherwise make  alternative arrangements to accommodate  the completion of the job to their  satisfaction ..I wish  i could  say differently but what is done  is done and  now you  have to repair the situation.