Can you fit 15 inch wheels on a 2017 Toyota Tacoma sr 4wd with spacers

Will 15 inch wheels work with spacers to clear the brakes

 

You can make anything work
Honestly, I don't know how much you would need, I believe the 16" wheels barely clear the calipers, so I would think a 15" would need to be pushed out fairly far.
Why do you want 15" wheels?

 

To give it an old school wheel and tire setup look

 

Good luck.

 

Would need a spacer big enough for the entire inside of the rim to clear the entire caliper.

 

fair enough, nothing wrong with doing something different!
I would crawl under a front wheel and look,, and made even take some crude measurements to get an idea. Especially if you have 16" wheels, you may see what clearance there is.
You could also shop around for a used 15" wheel to try. Or find a place that has a good return policy, and just buy one to try.