I typically buy them in batches of $300-$500 at a time. Last trip I had $2,500 (which didn't last long between buying an annual pass, paying for a group of 14 people to eat at Biergarten and Ohana's, the TIW card, and our stay at the Polynesian).
I've found that not all the Targets in my area carry the Disney gift cards.
I like saving the money, but you have to be somewhat organized keeping up with so many cards. I would take only a few at a time with me to the parks and keep the others in the room safe. Having a Sharpie on hand helps too with keeping track of what cards are used and how much is left on them.
When we checked into the Polynesian (at the club level area) it took FOREVER to pay for the room. We only stayed here two nights before transferring to OKW so it was only like $460 left to pay, but those 9 cards took a long time to go through. At least the guy was friendly, but I felt bad taking up his time for so long when so many people were checking in.