No need to worry. Your light switches are called "push-button" switches. They are very common in older homes. I have them throughout my home and all are in good condition. I have had some wear out on me, and although I can't pick up replacements at the local hardware store or home superstore, there are plenty of places to get them via mail order. In fact, if you really wanted to, you could replace them with "normal" switches... :-).
What you DO want to make sure of, however, when purchasing the home, is that you have a good inspection of the electrical system. The fact that you have push-button switches may indicate that your electrical system is older as well. I'm not saying that's bad, its simply something you'll need to make sure is in good working condition.
My home was built in the 30's. Some of the wiring has been replaced, some is original...My feeling is that if it "ain't broke..." -- but, at the same time, we check on it periodically to make sure the insulation is still intact, etc.
So, don't worry about your switches -- celebrate diversity!