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.

There's these d8s that use numerals for 1-8 and pips for 1-4 repeated twice. With three different sets of symbols (say by adding roman numerals) you could have a d12 that also functions as a d6 and a d4, though I can't say I've seen one. Trying to put 3 sets of symbols on each die face may create readability issues though.
I think there's other dice out there that function as more than one die but I can't find them atm. There's also ones that are basically just a fidget spinner with various segments or sets of fidget spinners for each die, but you don't actually roll those so I'm not counting them.