If you buy the inflator with the "threadless" style cartfidge, you can buy the replacements very cheaply at any discount store that sells supplies for BB guns, i.e. walmart.
With some full size frame pumps, you can fit then along your left seatstay, with one end at the dropout and the other kind of wedged in the space near your seat where the two seatstays come together. On many frames, if you buy the right size frame pump it fits very well in that spot and stays out of your way. Sometimes the pubp rubs on the tire though, so you'd have to try it at the shop first.
Sounds like you may like the co2's, make sure to understand how to puncture the metal seal on the cartridge before, I don't generally puncture them until I need to blow up the tire, they tend to leak out over time, You can stuff a couple spare carts in your seatbag. You can mount the inflator on your seatube, don't buy some gargantuan bag for under your seat.
Usually, I like to ride in a jersey with back pockets, so I stuff the flat tire and empty inflator in my pockets, then keep the spares in the garage so I can re-stock when I'm done with the ride, it's easy to forget and not have the proper supplies when you need them the next time.
