Incandescent Equivalent for Bathroom Fan/Light
Isherwood's comment is certainly valid, but here's a rundown of the possibilities (that I am aware of).

It has an Edison socket

That's one of these



It will likely be an E26, which is your standard light bulb size. Anything that fits will work fine (unless you find an incandescent that exceeds the wattage rating on the fixture).

It has an orange connector thingy

I don't actually know what they're called, but they look like this



Since these are mostly used in downlights (recessed lights) you can find LED trims for recessed lighting in the appropriate size (4", 6", etc.) for your vent fixture.

It has a built-in LED

You simply need to wire it. No replaceable bulbs are required.

Other

You could find something that uses a GU10 or another type of bulb that isn't an Edison socket, but this category is largely the same as the Edison socket category. Simply find any matching bulb with wattage equal to or less than the rating.