Carefully examine the stock before you buy. In my experience, I think I have lost more C. sterbai than any other, maybe except for C. panda. Both are very sensitive, even for a genus that is sensitive to begin with.
Corydoras do not travel well, they do not like being moved, netted, new tanks, or fluctuating water parameters or quality. They are usually the first to show stress over chemicals and medications, so in tanks with corydoras only medicate if absolutely necessary, and then watch them carefully.
Someone mentioned C. panda earlier, while suitable in size they are not good with betta because betta prefer higher temps than C.panda can tolerate long term. This species comes from mountain streams in Peru that are slightly cooler (low 70's F) and do not last long in tanks over 78/79F.
C. sterbai is a much better choice, one of the few cory species that can tolerate higher temps well, thus making a good match for Discus
and betta. But pick them carefully; if there are any signs of distress in any fish in the tank, forget them. Stress brings on internal problems, immune system issues particularly, and sometimes it can take a few weeks before the fish suddenly dies, other times it is sooner. In transport from farms to the store they are often packed too many in the bag and being highly sensitive to ammonia and CO2, this often weakens them considerably. This applies to all Corydoras. Take time to slowly acclimate them at home, mixing the tank water in with the bag water bit by bit, then net the fish into the tank (never pour bag water into a tank, even a quarantine). If the tank is new (within 1 month) it would be best to wait for 2+ months as they do better in established tanks where the water is more stable.
I had reason to use quinine on a tank last autumn, and I lost 2 corydoras solely due to the medication, they did not have the infection I was treating; and both were C. sterbai. It is also the species I have had barbel degeneration with more, only a couple in 15 years, but always they were C. sterbai. And this species is now commercially raised, not wild caught. Some think this should make fish more resilient, but it does not. In fact, my wild-caught C. sterbai are doing fine, it was the tank-raised ones that perished.
Hope this doesn't put you off; this is a delightful cory species, and with patience and careful handling it will settle in nicely and be very active.