Welcome! I like yard sales, thrift stores, estate sales, and such. You may try placing an ad on craigslist or freecycle if you have one in the area. Don't overlook good fabric that may have been made into curtains and tablecloths and such. Do you have public bulletin boards at your grocery store?
E-Bay used to have some great sales, but shipping prices have driven prices higher and higher, and dealers chop fabrics up into tiny bits and charge more.