I believe low potassium levels can cause constipation (among a million other things, of course). You can get potassium over the counter, but in stupidly low doses that's really not worth it. I try to drink a can of V8 Low Sodium every day (the low sodium has more potassium because they use potassium to replace the sodium and keep the flavor the same). Apricots are another good source of potassium. And of course, there's always bananas.

In the US, the FDA just finalized the new food label requirements that include listing potassium, so you'll still see some labels without potassium, but more and more will have it on there.