I found the vast majority of my sweaters at a church garage sale. The thrift stores just never seemed to have them, and when they do, they're too expensive to cut up.
I mostly find ones from GAP (not by choice, it just seems that wool from GAP is really popular around here!). I have to wash them two or three times with towels to get them to felt really well, and if they seem thin, I sew an extra layer in the middle. Work just fine, and super cute!
I looked at
this and it was really helpful. That diamond shaped piece in the middle is REALLY helpful, as it gives them a bit of extra room though the crotch area. If it's not there the tension will eventually pull the stitches in the middle and pull the wool apart. ( I used a pair for six months before it was a problem though)
For the first few I made them without that diamond piece just so I could get a feel for what I was doing. Once I got more comfortable making them and cutting up beautiful sweaters, I started adding that piece in.
I've
got a picture here of some I made. The pink one and striped ones are from wool sweaters.