All pet shops get a list of fishes available to them from the fish importers but most fish never get bought by the shops because they don't sell. However, these fish are usually still available to the shop, and most shops will order in fish for you.
Basically you should visit a number of pet stores and ask them what they have available on their wholesale lists. If they don't have the fish you are after, put your name and contact details down in their customer order book and get them to order the fish in for you. Visit the shop each week or every couple of weeks and make yourself known to them. And keep checking with them to see if the fish are on the list.
A lot of unusual catfish are only available for a few months each year because they are wild caught. So be patient and friendly and the shop should order you some in when they are available.
The other option is to contact an aquarium society in your area and see if anyone there has those fish, or knows a store that has them.