My understanding is that chain guards are usually best to get with the bike. If you have a front derailleur, finding something that works is...not terribly likely. There are reports of aftermarket stuff working, but they often included "and then I tried the other size, and then I got out the dremel tool..." and maybe it can never be made to fit at all. I think the SKS chainboards" and the ogk chain guard are the only things that will work.

Internally geared hubs, or bikes that only have a single chainring in front, are much easier to fit an aftermarket chain guard for, so if that's what you have, you may be in luck.

I just have a couple of those little velcro straps that live in my panniers.