Rapid Fire Plus shifters are great. I have them on my former everyday riding bike and with 6 years and about 12,000 miles of riding they have worked like champs. Only maintenance was the initial heavy WD-40 bath to get the pawls working and then a less drastic WD-40 treatment about 2 years ago.

If you go with friction thumb shifters I would try to find an old set of Suntour ratcheting models.