I don’t have enough used-transmission finding experience myself to know. The one time I had an automatic transmission problem — on my Ford truck w/a C4 transmission – I paid a local independent transmission specialist to have it rebuilt. So I ended up with the same transmission, just the internal wearing parts and seals were replaced. That worked out well for me. But how well that would work for you, don’t know. Modern automatic transmissions are a complicated beast. It really takes an experienced transmission expert mechanic to make that kind of judgement.
One idea, you don’t have to find the transmission local. The junkyards have a used parts network, and can find you one in another state and ship it to your local junkyard.
The transmission is more than likely dying. Age also has something to do with a transmission’s longevity.
With age the rubber seals in the govenor, clutch packs, and so on degrade. This means loss of fluid pressure, slippage in the clutches, and which in turn cause slow engagement, sluggish shifts, etc.
As for a used unit, I would try to avoid those unless you happen to find a very low miles (verified , good luck with that…) unit that was pulled from a wreck and stored.
It’s been my experience that about 35-40% of salvage yard engines, transmissions, and rear axles have issues from minor to sometimes junk status. Those gambling odds are too high to take a risk on.