My go to is whatever I have free in my freezer, then I buy the cheapest chicken I can find if I run out. Salmon parts, chicken bones, chicken backs, duck parts, clam guts, rockfish parts, scraps from herring and extra anchovies, I don't throw anything away as it is all good crab bait and free. I'm sure my chest freezer has paid for itself many times over just storing free crab, shrimp and halibut bait.
It's a humpy year, so late this year and all of next crab season will be humpy parts for sure.