Its only a 1 wire mod, and that one wire is easy to do (compared to say the points on a PSX, DC or PS2).. Basically if you know how a soldering iron works, you can do it..
Only place I know of selling pre-modded units is playthegames.com and they are UNRELIABLE! AVOID THEM.. I'll shorten my usual rant and say that after not recieving the saturn mod I ordered, and being ignored on email and phone, I decided to drive to the 'retail location' (which is only about 20 minutes from where I live), and it DOESNT EXIST. The address is for some chinsey looking paintball supply shop. I went in and asked the guy about PTG.com and he played dumb (my guess is he or one of his friends runs the thing out of their basement, hence the craptastic web design).. Better Business Bureau cant help me because its not a real business. So all I can do is post this as often as I can to try and spread the word.