I usually buy the e-GC online. I think the largest denomination they sell is $500 (but I could be wrong because I don’t think I’ve bought over $300 at one time). You can go to DisneyGiftCard.com to combine cards but can’t have more than $1000 on one card.
You get 5% off with your RedCard. If you have a RedCard Credit Card check your credit limit before you go, they can be stingy and people don’t notice until making large gc purchases.