Microsoft gift cards to purchase Series X despite being from different retailer

gift card balances are not transferable to a third party

You can only use your Microsoft account balance through the Microsoft store. You wouldn't be able to use it through a 3rd party.

I know some have, but they've largely been sold out and selling them online.

Do physical store locations sell XBOX Series Xs?

Hi there, PhoenizMi12;

Right now, the only way to add money to your Microsoft Account is via a gift card purchase and redemption. You can purchase these gift cards from any retailer where you would have previously purchased Microsoft Point gift cards in the past. If you happen
to still have MS Point cards, you may still be able to redeem them and have them automatically convert to your local currency. I'm not sure when this option stops being viable, however.