I normally order extra with whatever GB I picked up the board from. I only use 40% keyboards, so it tends to be rare that they are freely available in a keyboard shop, least of all the UK. Last one I remember was monorail, I purchased three of v1 (direct) and v2 (mechboards) each.

Sometimes you get third parties making new versions that are often re-imagined layouts for the same board or a continuation of an out of print board, for both see monorail for both. Some of the fun ones convert an existing layout into something else, like the wolfjaw xl for the equinox xl. They tend to be advertised by the 40% discord.

If you like a common layout like 60% they tend to be a lot easier to find, as long as its not been heavily customized. Mechboards have a bunch in stock right now