https://doublesixdice.com/collections/triplefours
These are the ones I started with. I wish they had focused more on the d4 line than the d6 line but that’d be like asking Wizards of the Coast to make good martials

https://www.diceemporium.com/product/roman-d12-12-sided/
Chessex ones available in a few variations with roman numerals

https://www.thediceshoponline.com/specialist-dice/d12-specialist-dice.html?dice_sides=100&product_list_limit=100
pipped 12 sided d4s, a bit more expensive

https://www.nobleknight.com/P/2147622485/20mm-d4-12-Sided-Metal-Dice-w-Black
And if you really have money to burn, these are the only metal ones I could reliably find. It’s a chonky boy, bigger than a d12 you’ll normally get, basically the size of a d20, maybe even a bit bigger.

There was a game a few years ago called Nibiru that apparently has a decently priced set of 6 metal 12-sided d4s with cuneiform symbols, but it doesn’t seem like anyone in the US has the dice in stock (I found one website but I’m pretty sure it was gonna scam me). This appears to be the publisher’s website from the UK, though it has had that message about restocking their warehouse since at least this past July so idk, but they say the dice are in stock. Searching on Google were also some Spanish and German sites that maybe had them in stock, but I have no idea if those sites are reputable.