I have a tempstar furnace model # FBF075B12G1. The furnace is maybe 3 years old. The little motor which turns the exhaust gas blower feels a little hard to turn. There is some rust on top of it. It will sometimes fail to start, which keep the burners from kicking in. If I spin it by hand, it will start, and the furnace works OK for a couple of days.
What do you think is a good way to fix it? Lubricate bearings? Replace bearings? Replace the motor? Replace the whole blower assembly? Where would I buy these parts in Naperville, IL?

Replace the assembly. Look in the yellow pages under heating.

Only 3 years old, it shouldn't have come close to going out yet, but - -

Those motors usually just have oilite bushings instead of ball bearings and when those bushings go dry the motor drags from the extra friction
When the motor shaft turns. You can check but they often will not have holes so you can oil the bushings.

That means a new blower.

A number of years ago when mine went bad I was not aware they were widely available online, either through discount HVAC supply places or over eBay. I paid over $200 through the dealer, THEN I found one on eBay for under $100 - Same THING.

Write down your furnace brand & model info and start looking online. Just do a search for i.e. "furnace blower" or "furnace parts" and you'll find
Lots of places that have them. You can email most of them and tell them your furnace info and they can tell you what you need. On eBay you can do the same, but there are usually good pictures so you can compare with yours and often they'll list furnace models they'll work with.

Usually fairly easy to replace, just 3 or 4 mounting bolts (don't tighten too tight or they can strip easily), the outlet exhaust connection, probably a plastic tube to a pressure switch, and a couple wires. Be sure to disconnect power first! Also be careful when removing your old one for the gasket between it and the furnace, take it easy pulling the old one off. If the gasket rips it can easily be repaired with some hi temp red silicone you can get at any auto parts supply.