Although many party shops and toy stores sell balloons for twisting balloon animals, the ones sold in the stores are generally not very good quality, are older, and will often pop (leading to frustration!)
As mentioned before, I suggest buying good, quality balloons (I’ve been using the Qualatex brand and usually order mine online.)

If you want to simply buy a bag (of 100 or 250), you may want to buy it off to save on shipping. (See links below.) There are different sizes of balloons, but a bag of 260 balloons are good to start with. I’ll add a page for suggested balloon products at some point.
If you want to see what other balloon products are out there, check out They are a pretty well-known company with a large selection. If you are buying A LOT of stuff, seems like a well-priced shop too. I’ve ordered from both.