You can find Baltic Birch at lumber yards. Call around first. It comes in 5x5 foot (60 x 60 inch) square sheets. Specialty woodworking stores like Woodcraft and Rockler sell cut pieces, like 12 x 24 or 24 x 30 inches, for example. There may be a store near you. Baltic Birch is expensive from them:
There are different grades of Baltic Birch. If you price shop, be sure you are comparing the same grade and thickness. Baltic Birch is actually metric. Stated Imperial dimensions are approximate.
Around here (Southern California), you can also buy "Russian Birch" plywood. It is similar to Baltic Birch (many thin plies, all birch, no fills). It comes in 4x8 foot sheets. Prices are usually lower than comparable Baltic Birch, but I have seen them cost more, too. (My price comparisons were "per square foot" for the same thickness plywood.) I have not used it myself.
When I got back into woodworking a few years ago, I bought some low-cost plywood just to have some in stock for jigs and stuff. Based on my previous woodworking experience from the 1970s and 1980s, I expected something flat and practical. What I got was badly warped and useless. I went out and bought some Baltic Birch. It cost twice as much, but I never looked back. I like to keep Baltic Birch "in stock" at home - 1/4, 3/8, 1/2, and 3/4 inch. When I run low on a given thickness, I replace it with another 5x5 sheet.