Home Depot has both T12 and T8 LED bulbs that go into Florescent fixtures without modifying the fixture. Just plug 'em in and turn 'em on. They do require that the fixture has an INSTANT START ballast however. They are about $7.00 each. Haven't tried them yet myself.
“We should be careful to get out of an experience only the wisdom that is in it and stop there lest we be like the cat that sits down on a hot stove lid. She will never sit down on a hot stove lid again and that is well but also she will never sit down on a cold one anymore.” - Mark Twain