I came up against this problem too, and I especially had a problem since 3/4 of my major professors are now retired (and since I'm a non-science major a couple schools required a letter from my major, not just non-science).
Just make sure you satisfy what the school requires. You can always call and ask too. Also, beware of the mandatory supervisor letter some schools include if you've been out of school and in the workforce for awhile.