It really doesn't matter, unless you are looking for a set of springs w/a already built in them. The first set I used came off of a Suburban, the others have been from a "backyard Recycler" who had several sets of springs just labeled Chevy. IIRC the last set was a 4 leaf pack. You could also pick up a set of Toyota rear springs and use them the same way, just cut the Toyota main to fit under your current one.
Are you working with stock springs or after market lift springs?