I've been thinking the same thing, but navigating the DD-WRT Wiki page of compatible models is an exercise in frustration. That's largely the fault of the hardware vendors, of course, who release a bewildering array of short-lived landfill-destined models, often under similar names; but after an hour or so of research I still hadn't found one I could order online that I was reasonably sure I could flash with DD-WRT.
Obviously it's possible - I know people running DD-WRT - but the handful of old routers I had lying around don't seem to support it, and I hate to buy something for the purpose and then discover it won't work.