Well -there's "good news and bad news"... The Monza used a 4 lug on 4" pattern and you should be able to find them at a local tire and wheel store. There are some available on ebay and such - 4 LUG Centerline Wheels Chevy Vega, Monza, Others | eBay

However - to Gary's post, long term the fix is to have your axles re-drilled to accept standard 5 lug components. I have a 1963 Nova SS that was four lug and I converted to 5 lug to make life easy and be able to get some "real wheels". Mine was easy as the rear end was swapped from a 1965 Nova and I used a Fatman Fabrication kit for the front end. It was however; an "investment" of about $4K as I rebuilt rear end, converted to 3 leaf springs (from mono) and went with killer shocks and traction bars. the front kit required me to buy MacPherson struts and I used 11" rotors from a Z28.

Sorry for the brief "hijack" - but hope you find the additional information helpful for future planning as it sounds like you want to continue to enhance the Monza.

Regards,
Glenn