Wm. Booth, Draper, sells wool tape, used not only to protect hems, but (per their suggestion) as garters. You just wrap a piece around the upper leg and tie a bow knot. Most of their stuff is 18th century, but things like the wool hem tape are good for the 19th, too. Needle and Thread in Gettysburg also carries the stuff. As one who has circulation issues, I recommend not tying it too tight! The wool tape, unlike ribbon, is fairly elastic.
I've seen garters carried by sutlers, but most of them are thinly disguised modern elastic.
Assuming you don't lift your skirts too high, a piece of wide elastic (cheaper from your local chain fabric store than from the sutlers!) should work. If you are also wearing drawers, the tops of your thigh-high stockings, with garters, will be well hidden by the drawer legs as well as skirt and petticoats. Again, not too tight! The argument for tying rather than sewing it to fit it is that you can loosen it if necessary.