Walmart, 3 bucks,convex 95mm close works good not a perfect fit tho.. same thing at many auto stores. At most auto stores you can get a blank for 4 bucks,not a convex you can cut with scissors to fit. I have had no luck this side of the pond for 100mm. The euro guys don't seem to care
I did find one convex on a plastic pedestal that rotates,it was 3 bucks and you can rotate it easily on the fly with a gloved hand,but the tape was weak. In the end I might get some good tape and put them on both sides, the rotation allows for fine tuning very quickly. I find it allows for those baggy pant days 
