TBH, best place to start is with a flawless paint job....
Sponserships dont happen as often as you would think...lots of people just 'rep' garages/companies who help them...getting freebies out of companies isnt as easy as it once was
The worst bit about most of it, is you will need a substancial following on social media...and I means thousands if not tens of thousands of people
Be prepared for the big companies to already have a big social following too so I highly doubt they will give anything away for less than trade price
These days, unless you are building something out there compared to the norm, or have built something so you are known by tons and tons of people...dont expect too much
And being a bodyshop company, you also have to stand out compared to other 'flawless' paint jobs out there...you need to find a niche or something you are amazing at and can battle it with the best in the business....maybe something like custom arches, metal work, filling work, bodykits etc