Hi SAK,
First call them Mags or Magazines. Your carbine can use both 15 or 30rd mags.
30 rd mags will work best with the late style magcatch with the support arm on the left. Pretty sure these were called a Type IV. It just takes a minute to swap out the Magcatch.
They get pretty heavy fully loaded......
M1icon Carbine, M2 magazine catch. USGI. : Other at GunBroker.com


Eddy has a good buy on some commercial 30rders that work very well. Hard to beat the price also....... and you'll save wear and tear on collectible magazines.
M1 Carbine, (4) NIW 30 Round magazine. M2 : Magazines / Clips at GunBroker.com


Here's a list of his auction items. TYPE IN M1 CARBINE MAGS IN THE SEARCH BOX BELOW WHERE IT SAYS ... SEARCH FOR OTHER ITEMS FOR SALE BY 81MM.
I can't get just his MAG PAGE to show up.
GunBroker.com - Auctions by 81MM


Here's a really nice one for the $$.......
M1 Carbine WWII 30Rd. MAGAZINE : Magazines / Clips at GunBroker.com


For other GunBroker sellers, type in the search box.... M1 carbine magazines.

HTH,
Charlie-painter777
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