WHERE CAN I FIND A SUITABLE DESIGNER PROM DRESS

I don't know what you mean by suitable, but there is a store on the southeast corner of 6th Ave and 37th St with lots of them!

For upscale designers, I'd try the high-end department stores (Saks might be a good place to start, or Barney's if you've got the budget). Or, try their stores/showrooms directly. It would depend which specific designer you're interested in.

For lower-end designers, try Macy's or Lord and Taylor. You'll find lots of nice (albeit non-designer) gowns there as well.

I think askanewyorker might be referring to Elite Designs (990 6th Avenue at 37th Street) which is a wholesale/retail evening wear store. Right nearby is Elle Belle (1020 6th Avenue at 38th Street) which is a similar type place. Both have lots of gowns to choose from, but don't count on finding designer names there.

Hope that helps!

Filenes Basement

Or better yet check out the consignement shops on the Upper East Side. Rich women wear them once (if that) then get rid of them. No one needs to know its "used".