You may be able to get them from your church or school and help out with fund raising at the same time! I have bought them through our SCRIP program at DD school several times. They usually don't keep Disney on hand but it only takes a week to order them.
I don't know if this varies by state, but here in IL all GC are tax free-you pay the sales tax when you use it to buy items. You're not really "buying" anything when you buy the card.
I buy them at Kroger to get the gas rewards. Around Christmas time many grocery store around here also have a deal where you $10 back or off when you spend $100 on GC.