If you're lucky you may find New Old Stock pieces of artwork for your game for sale on the collecting newsgroups or on eBay. Most likely it'll be very expensive. There needs to be an alternative. This is a collection of high quality digital images of various artwork in a format suitable for professional printing. You can download these images and take them to your friendly neighborhood printer on a disk.
Many printers can now print on large, oversized, high quality inkjet printers that look great.
They can print on adheasive backed material for sideart or material ment for backlighting for marquees. If you need to print a control panel overlay, look into having it reverse-screened onto lexan. You can print them out on your personal printer at home with good results, too. I use an Epson Photo 1200 to print art. The quality is great and the printer is quite reasonably priced.