I've seen white sapote posted on Etsy for the past two years in a row around mid-summer. Last year I bought 3.5lbs for $37.90 in late July.
Abiu I've only seen available for pre-order on MiamiFruit. Apparently the waitlist is so long, you may end up waiting a couple years before your order is fulfilled.
If any private abiu sellers are reading, jot me down as someone else who is very interested in buying some.