Gas Station Numbers
Vintage gas station signs are great for being able to see numbers at a distance. There are actually quite a few different fonts and colors available. I happen to like red numbers on a cream background, but you can also find red and white, black and white and navy and white.
Etsy is a good source for these vintage signs and you can find them from $10-$40. There are quite a few sellers that have them if you do a search on the site. Here are some sellers with these numbers for sale at the time of this posting: 26 Olive Street, Cousinm's Shop and Blue Bell Bazaar.
Another great source of large numbers are numbers from old theater marquee signs. You can find these in metal or plastic in all different fonts, sizes and colors.