Non-SSP species especially are often acquired from outside the AZA. I'm aware of zoos getting animals from a number of different non-AZA sources, such as:
- Reputable breeders, especially for various herps
- Surrendered pets, both illegal and legal exotic pets sometimes find their ways to zoos. Both in terms of legal species commonly used as ambassadors (e.g. ball pythons, bearded dragons, etc.) and also more exotic species (e.g. alligators, armadillos, tigers, etc.) that may or may not always be legal.
- Wildlife placements, plenty of native species make their way to zoos as non-releasable reintroductions. Almost, if not, the entire AZA populations of black bears, grizzly bears, bald eagles, and cougars are sourced this way- along with skunks, opossums, native owls and hawks, etc.
- Local colleges and universities, especially in terms of farm animals raised in college animal science programs (e.g. goats, alpacas, rabbits, etc.)
- Non-AZA sustainability partners that participate in SSPs. Non-AZA zoos can apply to be an SSP sustainability partner and participate in an SSP. Alaska Zoo does this, along with some recently accredited zoos (e.g. Columbus), probably some others I'm not aware of.
- Captures from the wild, for aquarium fish (unfortunately).
With the new SSP designations, I wouldn't be surprised if it becomes more common for AZA zoos to work with places outside the AZA, as less species are going to be formally managed and subject to the rules related to SSPs.