A knife handle is a rather small piece of wood. You are looking for quality rather than quantity. I have found that small scraps of excellent wood are usually available for free, or at a very nominal cost.
Home Depot, Lowe's, or any lumberyard where they sell hardwood will usually have a pile of cut offs availabale for very nominal cost, or even free. I've noticed the big box stores don't sort the wood, and I've found some really nice tiger maple mixed in with regular stock.
Distorted grain makes for an attractive pattern, but is often difficult to machine by modern production methods. The local hardwood milling company discards pieces with fantastic grain patterns. Also ends and cut-offs too short for any other purpose are easily long enough for knife handles. They charge $30 a pickup truck load for their Maple and Oak "firewood".
Likewise a local ruler company charges a nominal fee for their Basswood (good for sheath liners) and Beech "firewood". I'm told that venitain blinds are often made of basswood as well. These would be just about thick enough for sheath liners.
A local furnature factory discards any turnings that are not perfect. Since they have to pay to have the scrap hauled away, they are happy to supply all the maple chair and table legs you wish. I heated the house one winter with the discards, and have a lifetime supply of tool handles for files and chisels.
Fruit wood (Apple, Pear, Cherry, etc.) is a true delight to work. It has a wonderful mellow texture and color. Historically it was a favorite for knife handles and gunstocks. It is often available free when orchards are cut down or pruned.
Packing crates from the tropical countries are often made of rough cut exotic hardwoods. Is there a motercycle shop near you?
Discarded furnature can be another source. A leg from a broken maple or cherry table will make six or seven handles. Old furnature will of course be perfectly seasoned. Broken gunstocks yeald walnut knife handles. You may be able to recycle a damaged family heirloom.
Don't overlook the source, trees. Is there a recently fallen hardwood tree in your area? Dead and standing trees will be well seasoned, and are seldom rotted. If you can take a section from a crotch, you will be guaranteed an interesting, tight, grain. Bringing home a piece of surplus firewood from a campout or hike can be a way to evoke treasured memories whenever you use the knife you make from it.
These are just a few ideas that have worked for me. Use your imagination. Happy scrounging!