Join Date: Jun 2012
Location: Fort Collins, CO
Couple of options for co2. If it's a paintball tank, you can fill it at either Sports Authority in town. For 5/10 lbs tanks, you should be able to get those filled at Jax (which will also fill paintball tanks).
For plants, locally is really just D&G, although the packaged plants at Petco and Petsmart are pretty nice, if you pick a non-melted package. I've had great luck with their assorted crypts, and fairly good luck with the rest. Great White doesn't tend to have much variety the couple times I've checked. The new place opening soon(?) is an unknown.
For online the only place I could recommend is the "For Sale/Trade" section of this forum. I've had good luck ordering plants from other hobbiest via the forums.
There's also a few local members that may have some stuff for trade (I don't really have anything at the moment or I'd offer!)