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What's available?
Steel or aluminium.
Not custom.
Ta
If you have old straight steerer forks you can make them fit pretty much any frame... Why limit future fork purchases?
As above tapered forks seems easier to source than straight. Id go tapered and space current fork to fit
SIngual Swift still has a straight steerer I think.
Salsa El Mariachi will take both.
Gen 1 scandal? I just sold one, there's still a good few out there and they're really good... And cheap!
As above, probably just the Swift if you want to buy one new.
Second hand there's probably a bigger choice, but even so 29ers and tapered steerers came into the mainstream within a year or so of each other, so your choice will be limited somewhat to a few older frames.
Any particular reason why it can't be a tapered frame?
Strictly speaking the El-Mariachi (an others) isn't tapered, its 44mm, which takes a internal headset for straight steerers or an external one at the bottom for tapered.