Amazon has a 20" fender for $5.85 right now, qualifies for free shipping. I bought one for the rear tire of my bmx. Hopefully it will keep my shirt back clean going through puddles... not like it ever rains here.
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Planet Bike Hardcore Front Recumbant Bicycle Fender with Stainless Steel Hardware and Mud Flap (Black, 20-Inch Tires/50mm Wide)
Theres some other sizes too, but for a little more cash. Anyway I thought this was really cheap so I wanted to share.