the 12 inch tires used on Geo Metro's years ago were a size of 145/80x12 The newer generation of Metro's from 1995 on, used 155/80x13 which are still fairly common - can't say so much for the 12 inchers.

W-M is still my source for the 13's I use on my 2001 #7 Swift (main daily driver since 1987 - 7 of them so far = over 1million miles driven to/from work - now retired = still a 'great' cheap daily driver, over 43+mpg and $$$ for!!!!!) These 13's at W-M run about $37 per and last me almost 50 miles thou each - I use the same brand/model of tire on my teardrop = 205/75x14 and $$= $57 each = they ride GREAT!! and have been wearing just 'fine' (and are USA made - by Goodyear for W-M-with the "Douglass" brand name on them) I have seen trailer wheels for sale at Tractor Supply and if i remember right, I think I did see a 4 lug version of a 13 inch wheel there?? You might do some fender sizing to see if you could fit the 13 inch version.......... FYI: a 155/80x13 will measure out to 22.76 inches in diameter / 145/80x12 = 21.12 in diameter = 1.6 in larger in overall diameter for the 13 inch tire/wheel setup, thus putting the tire .8 inch closer to the fender ............. (might as well say "one" inch closer......)