I have a heavy duty 3/4 ton 2001 ram van ( B2500), and I searched for a lowering kit also with no success. I did find a set of 2 in. lowering shackles for the rear
http://www.streetbeatcustoms.com/gfc91146-001.html they took a little modifications to install but they worked well. If you do a spring under axle modification for the rear as briefly described by relayda, keep in mind that this mod will lower the rear about 5 - 7 inches depending on the workmanship, depending on your tire size and rim width you will probably experience some tire rubbing, suspension problems such as bottoming out and shock absorber length, also some driveline vibrations (especially at higher speeds) due to the pinion angle change unless this mod is done by a professional.
As for lowering the front, I tried the only lowering spring I could find which was a 2 in. lowering coil for a 1 ton dodge ram pickup. No success, didn't fit. I simply removed the original coil and cut 1 full coil off the bottom of the coils. You have to cut 1 full coil because the top and the bottom of the coil seat in fixed notches in the coil buckets. I also shaved 1 in. off the bottom bumpstop. The van rides very nice, has a firm feel, doesn't bottom out.