Ebay is probably where I buy the majority of my fabric. Most of it from private sellers – either folk destashing themselves or selling things from a house clearance.
You can find a group local to you by searching under groups on Facebook for ‘fabric destash’.
Most quilt shops, either in shop or on their online store will have a clearance section for remnants, off cuts or bolt ends (sometimes called bale ends or roll ends too).
If you like buying scrap bags and bundles you can search for those on Etsy and find specific listings for that from a range of sellers.
Generally these bags are rolls of fabric strips similar to a jelly roll – they tend to come from one fabric line and be roughly 2.5″ wide but not always exactly that size.