If you just want something cheap you can wipe off and don't mind black color, check home improvement stores for vinyl runner. You can get a piece around 25-30" wide in a one foot length for something larger than 12"x24" for like $2-3. Another low cost option, try hitting up auto parts stores and see if they have any surplus counter mats laying around. They may be willing to give or sell you one, sometimes they have stockpiles of them from various vendors or with their own marketing branding.
Places like office depot have 22" x 17" desk mats made of translucent pvc (easy to wipe off) for around $15. Amazon has supersized 'gaming mouse pads' or desk pads that are 35" x 16" for like $13-14 but I'd imagine they're some sort of foam with fiber covering. Not sure how they would hold up to spills, how easy or difficult they'd be to clean off.