Isnt Decatur in Michigan? Anyhow if you are close by toward the Chicago IL area, there is a place called "Soko International". They have alot of red tops S13 SR20's and some black top S14. Cheap, last time when i was there sourcing some parts for my car, you can pick one up for about $1500 with a tranny, ecu full loop and etc. S14 usually run a bit higher well over $2k. On the other hand if you want a clean SR20 motor then contact Karim screen name here on forum "Racer98" he can get you a cherry motor just like he got me my RB26 and you wont be dissapointed at all.
As far as buying a build up motor right from scratch could be a risky. There is a shop close by my area that has a bunch of SR20's in stock that they could use for a build up, cost wise is around 7-8K depending on what u want out of it, that was atleast their quote for me on fully build motor with a T-78 turbo.
My sugestion to you is probably geting a clean SR20 motor w/e it is S13, S14 or S15 and moding it as you go along.