It's usually an O-ring in there, but it all depends on what brand TB it is. But generally yes, you can remove the throttle body blade from the shaft, and pull the shaft out and you should see a seal or O-ring in there
Sorry I don't know the exact size but I always have a box of orings from Amazon or Harbor Freight with a ton of different sizes and they always seem to work. Just make sure you get the rubber that's compatible with the fluids it might come in contact with like fuel or whatever.
I don't know the size. Would have to pull a TB apart and measure to get the dash number. O-rings are standard sizes so just a matter of matching it up. Also depends on what brand. This a stock one?