We just pick them off our lemon trees out back. If we didn't have lemon trees, my next choice would be buying some from one of several folks walking around selling lemons to be able to buy a pacha (small 365ml/12oz plastic bottle of guaro/licor) or two. If those 2 options failed and the local super didn't have any, I do recall finding lemon juice at a large super in Liberia a few years ago. I think it was at the Jumbo back then. We went through a lot of lemons when I had the bars and at one point I fancied ourselves as making and selling real cocktails so I had to track down more lemon juice than we could squeeze. Of course nobody wanted to buy cocktails and after a few weeks, I sadly closed my Old Mr Boston Bartender's Guide, and we went back to selling Imperial and Pilsen. The closest we ever came to selling cocktails was the occasional Rock Limón or a michelada. I think I recently saw lemon juice at Mayca, which last year was bought by Sysco -- so they may rebrand at some point.
If you can't find any, I'll bet you could ask your local bartender to buy some extra when they buy them. Surely they buy them every few days or so from someone.
Good luck finding some!
Sam