L.,
Check out a mom-to-mom sale in your area. Today I'm going to one at Duncan Elementary at 26 Mile Road.
Also look on Ebay for clothes listed in a bundle or lot. Search for example: boys, 3t, lot, Gap. You'll find some of the better clothes if you use a well known name brand. I've purchased some very nice, gently used items for $1-2 including shipping charges.
Also, as the weather gets warmer there will be plenty of people having garage sales.
Look for someone in your area that you can exchange clothes. I had a friend from our church that I gave her my daughters clothes (she's 6) and she gave me her son's clothes as he outgrew them (including shoes). It has worked out really well for us and we continue to share.