I don’t know if you have a pet at home near you but they sell them a group of 3 for £10 whereas other fish shops are a lot more. My local pets at home have really healthy ones as well.
they have albino, copper and pepper ones but the bigger the store the more they have
Find out the scientific name of the Corydoras and ask your local pet shops if they can get them in for you. If they don't have them available straight away, see if they can take your name and number and put it in a customer order book. Then they can order them in when they become available. Pop in and talk to the shops on a regular basis too just to make sure they don't forget.
Alternatively, contact a local fish club and see if anyone there has any. Or see if there is a catfish club/ society that has them.