"Custom" and "Affordable" are kind of mutually exclusive.

You have a choice in this hobby between getting mass-produced, off-the-shelf items, at an affordable price, or paying a premium for rarity.

There's a bit of a middle ground available, depending on what you consider "affordable".
You can look at places like AliExpress, see what they have available, then look into piecing together your own creation, using the parts you find.
That will still cost you considerably more than you'd spend on a budget board, but will keep you out of the bespoke price range, while eliminating the "out of stock" problem at the same time.