You've certainly got the right idea - cream of tartar is nothing more than an acidifier and so any acid can be used as a substitute. It combines with water to create tartaric acid.
Two important characteristics of cream of tartar are that it is (a) dry and (b) mostly flavourless. Vinegar, lemon juice, etc. are all wet and add some flavour of their own.
The substitution ratio most often cited for baking is 3:1 - that is 3 tsp vinegar or lemon juice for every 1 tsp cream of tartar. Don't use the 1:1 ratio typically given for egg whites (meringue) - the whole point of this in candy is to actually lower the pH, not just stabilize a foam. Again, pay close attention to how much liquid you're adding, and if it's significant, adjust the water content accordingly.
(Note: Technically, lemon juice would normally have a lower pH than 5% white vinegar, but the actual pH of these is so variable that it usually ends up being pointless to worry about it. Consistency is one of the advantages of using cream of tartar.)
You could also use citric acid or ascorbic acid (vitamin C). Citric acid is far more concentrated than lemon juice, you would only need to use 1/4 (25%) of the amount of lemon juice. That ends up being about 3/4 tsp citric acid for 1 tsp cream of tartar. It's a much closer substitution and doesn't change the flavour much. I believe that ascorbic acid is the same. Both are normally sold in anhydrous (powder) form, similar to cream of tartar.
All that being said, cream of tartar really isn't hard to find, it's sold in the spice section of any grocery store. One little tin will probably last you many years, so just buy one next time.