Find a hobby/slab sawmill. the offcuts from making a cant (or the first or last section of a slabbed log) are well on their way to platter/shallow bowl blanks. Also, if you want to minimize waste, they'd be a good contact anyway if you have/have access to a good log, paying for custom milling can be worth it if you have more money than time, or otherwise don't want to deal with a bunch of chainsaw work. If you have a whole sawlog available, I've heard of some hobbyist-types doing the mill work for 50% of the lumber. If the log is free or close to firewood prices, you'd still net a ton of blanks with much less work.
A 4-5" thick trapezoid shape the bottom width of several inches and a top width of your target bowl size is ready to go right on the bandsaw, will be quick to rough with the parallel faces, cut down on waste (half the undercutting is done for you), and an 8' long slab of these dimensions and 14" wide can net you about 30 bowls of a variety of sizes if you core (with 7 of them being quite large), with the only chainsaw work being a few easy crosscuts. This is our favorite starting place for production-style turning work. "Mantlepiece" style slabs are excellent for bowls and other medium projects, too, and can be had green for pennies on the dollar compared to store-bought turning blanks-- especially if they start to twist a bit, they're kind of useless as mantles at that point.
Another caution is that with trees near dirt/gravel roads can be diiiiiiirty and or gravely. Wash first and avoid gravel-- you won't get the same warning signs from iron oxide, though in our experience it's usually stuck in the bark, so look carefully.
