Have you found bottled aloe vera juice without any additives at all? I haven't seen any without at least a little lemon juice or something like that (for stabilization & preservation). I've most often used the one from WalMart (linked above by
@dmcgee5034); it contains a tiny bit of citric acid and sodium benzoate. Since I'm a sporadic soapmaker, I like that it's sold in quarts as well as gallon jugs. Trader Joe's sells organic aloe vera juice by the gallon at a good price, too; it's made from aloe concentrate and contains a little bit of lemon juice. The Lakewood brand organic juice sold in my regular grocery store was the first I ever tried, but also contains lemon juice (and is way more expensive!). All of these have worked well in soap, and I really couldn't tell any difference between them.
Maybe mixing your own with the powdered aloe vera that
@justsomeguy mentioned would be the best way to be sure nothing else is added?
example of 100x concentrate
I'm not sure it's cheaper than picking up jugs from WalMart or Trader Joe's, though there would certainly be less plastic packaging to deal with.