To advertise on billboards you can do something called a “direct buy” where you call the number listed on the billboard to inquire about placements and pricing.
There are also vendors who work specifically with OOH (Out-Of-Home advertising) where they will find the billboard placements for you.
A newer option that I particularly like is Blip Billboards. They provide digital billboards at a really low cost (mainly because you are sharing the billboard with other advertisers).
Hopefully this help!