It depends upon what you want made.  The people at Poserworld have always been open to requests by members.  They also offer custom services.  Sixus 1 Media is another vendor who will do custom work.  Mr. Sparky at PoserDirect and Vashing Point are others.

Much will depend upon what you want and how much you are willing to pay.  There are many variables that you would need to work out with the vendor you are hiring.