Yes, simple literal 100W incandescents have gone out of style. There is a law, it is not enforced, but stores did not want to be stuck with a large stock in case the law was enforced, so they stopped buying 100W simple plain incandescents.
You may find 100W incandescent "globe" lamps, the big round things used around dressing tables. "Decorative and specialty" bulbs are exempt from the law. Little flame lamps are 7W-25W, the small Globe lamps are 45W-60W, but there is a Large Globe at 100W.
The "100W equivalent" *Halogen" (90W-75W actual) is reasonable for smaller amplifiers.
The >100W lamps are still widely available as simple incandescent. H-D sold me several boxes of 150W and 200W plain old incandescent for my garage.
At the chicken-feed store (also many hardware stores) you can buy Heat Lamp bulbs. Big reflector bulb with red coating.
Should be 250W-300W. There are also silly "energy saver heat lamps" which only do 150W. In chick-raising, this leaves your chicks cold; but a good size for general amps.
I *thought* that at 300 Watts (or above 300W??) the Code required "Mogul" sockets, which are larger than "regular edison" sockets. Large stores have mogul sockets but may not know where they hide them.