Honestly, if you buy a J-mod "kit" (i.e. new valvebody and/or separator plate already drilled) and have the ability to install that yourself, then you can save yourself the money and drill the holes on your own as well.
The basics of doing the J-mod:
1. Remove pan and drain fluid
2. Remove the valve body
3. Remove the separator plate from the valve body
4. Drill a few simple holes in that separator plate
5. Remove springs from accumulators (if you like, I left mine alone and am very happy with it)
6. Do whatever upgrades are necessary for the pre-98 accumulators
7. Reinstall everything
8. Fill up with fluid
9. Ask yourself why you didn't do this mod years ago.
The ONLY step that buying a valvebody or pre-drilled separator plate would save you is drilling the holes in your stock separator plate. And honestly, that is ridiculously easy. Read over the articles posted by Trombone and Chris, mark the separator plate accordingly, and drill the holes. It really is so easy that a caveman could do it. It took me maybe 10 minutes to do that step.
All in all, the J-mod sounds much more intimidating than it actually is. I was scared to death of it initially, but it took me less that 4 hours start to finish to do it, and with the write-ups listed, it really isn't that difficult. And as I said in step 9, I can't believe I owned my car for 4 years before I did this. One of my favorite mods to date, up to and including adding the supercharger.
Best of luck.