Can't help you in Great Falls - except for the usual places. Might try Rokeby off 123/GW Parkway. Haven't been by there for several years but I thought it still had boarding etc. Know of some places in Potomac - perhaps not too far from Great Falls. Know of one place that I think might have a stall available. Let me know if you need contact info for that place
I don't know that there's much around here any more by way of larger facilities with rings...most have been sold for development. Southdown, etc are probably full. I would definitely call the Saddlery on Walker Road - now owned by a woman named Sarah who was super helpful to me a couple of years ago when I moved my horse back home.