Some places to find 10x12 outdoor rugs are Overstock.com and Amazon. Overstock.com makes it easy to find rugs by going to the area rugs category and selecting the size.
To find outdoor rugs of this size at Overstock.com, click on the Home category, then on Home Décor, then Area Rugs. On the left-hand side of the results page is a list of the different sizes available. Select 7x9-10x14, and click on the 10x12 option. All rugs listed are within this size only.
Amazon does not have 10 x 12 specifically as a size, though it is easy to search for this size. Go to the Home and Kitchen category on Amazon, then click on Home Décor and then Area Rugs and Pads. Select All Area Rugs and look through the different sorting options. This includes selecting a color, style, material or brand.