You don't need a supplier, just a piece of channel/tubing/angle to bolt to two angle brackets welded to the inside of the frame rails. Bolt the cross piece to the angle brackets so it is removable in case the trans has to be removed and drill holes in the center in line with the 350 mount cushion. A one hour job if your slow.
Here is a photo showing the left side of the fabricated member, it's in a 52 but the cab is so far above the frame on an S10 conversion it should apply to all years. For the AD guys it also shows how to hook the emergency brake cable to the factory E-brake rod.
http://s32.photobucket.com/albums/d41/coilover/Scroll down to first picture.