I normally recommend Arcade Art Repo. I've purchased replacement marquees and art for my Astro / U4 and the quality is excellent.
https://arcadeartrepro.com/?s=blast+city&post_type=product
OG Sega ones do come up quite regularly on Yahoo Japan as well.
+ 1 for @themisterfalcon. Purchased a very nice Mr Driller 2 board from him. Very carefully packed and made it over the pond in just a few days.
Nice dude, very easy to deal with.