what size spacer do i need to put 18s on my 2002 chevy cavalier?

generally speaking, keep the stock wheels on any vehicle you ever own and avoid problems that changes like this can bring.

And if spacers were to be required, your wheel dealer would be the definitive word on those. If they didn’t even bring it up, knowing the application, that makes you wonder. Are they not needed ? Does the dealer not know what they’re doing ? Are the rims actually wrong for your car ?
I would not begin to mess with something like that, you’re only asking for future troubles.

You shouldn’t need a spacer if the wheels are correct for the car.

I wouldn’t do this. You’re going to overwhelm the brakes with all that extra weight, and spacers will put a tremendous load on the wheel bearings. This is asking for trouble.

The off set for the wheels is obviously not correct and having to deviate safely depends upon how much and a potential problem maker. 18’s an a Cavalier ? You had better be running on a race track all day, as any pothole is potentially a big problem. At the risk of sermonizing, I suggest you save up for car where 18’s are a dealer option.