I recently got a 78 camaro and it has a lot of rust. The rear subframe has rust holes in it. My mechanic said he can patch it but I was looking to find weldable sections of rear subframe or a full rear subframe. I haven't previously dealt with a unibody car are the part readily available for a 78 camaro?

 

There's lots of parts out there.
https://www.carid.com/1978-chevy-cam...;]=SUBMODEL LT

https://www.ss396.com/camaro/RFR-81L...SABEgIMavD_BwE

https://www.ebay.com/itm/70-81-Camar...-/173457582928

https://www.ebay.com/itm/Rear-Frame-...Model%3ACamaro

Or if you have lots of $$$$$ you can do really cool stuff way better than stock.
https://www.cachassisworks.com/cac_library.html

 

And you could go really crazy with something like the Roadsters Shop Camaro "Rampage" And for more stock application Places like OER.Classic indutries .Etc.If you can do it yourself or have a shop do it.