There are some postcard sets by gay artists, like Herb Ritts and Robert Mapplethorpe that can be found on ebay or museum gift shops.
There are some cute designs on Red bubble. I like ryegardens shop: https://www.redbubble.com/people/ryegardens/shop
Otherwise, I am lucky that the book stores and museums I’ve visited usually stock some LGBT cards. For instance, this is a postcard of a murual on display at the Berkeley Pacific Film Archive. It was made by a trans artist and is titled “Fake Affordable Housing for Trans Elders.” Because of the pandemic, it was on display for many more months than planned.
It’s always a hunt! It’s fun to imagine finding a beautifully designed LGBT postcard set Maybe one of drag performers or historicial photographs or the history of ACT up.