I bought 70% lactic acid off of Amazon a few months back, and I've seen lesser percentages too. You can also find Glycolic, Malic, and Acetic Acid at varying strengths on Amazon.
No wonder they own the online shopping world lol.
Any particular brand? Or does it not really matter?
It really shouldn't matter, I mean it is an acid with a defined chemical composition. Lactic acid from one seller is equal to lactic acid from another, assuming you're actually getting lactic acid and the other dilution component is not harmful. Just look for anything odd in the reviews before you buy, and make sure the seller has over 50 or so reviews, and you should be fine.