I had a little co2 kit that i bought from a LFS, cost me about £70 if I remember right, not cheap but thought it was perfect for my nano tank set up I had at the time.
Works really well once get the hang of this little set up. Main annoying bits for me about the kit was it has a manual regulator, this means having to turn this on and off manually and setting it right every day. And the little co2 bottles did not last too long, about 6 - 8 weeks on average and at £10 per canister, it is fairly expensive over a year to run.
If you want a cheap co2 set up, I would advise going with a diy set up, not as complicated as you think to be honest to purchase and set one up.
I have a 2kg Fire Extinguisher set up, with a dual gauge regulator with solenoid, means its plugged in mains power supply and with a timer it open and closes by itself without having to adjust settings every day. Makes life a lot easier. My DIY set up cost me £65 all in, sourcing parts from auction sites and fairs.
While I know you say you dont have the room for this type of set up, as DrRob suggests, you could use a paintball co2 canister which are pretty good and you can easily get parts to adapt to fit to regulators and solenoids.
There are a variety of co2 canisters sizes out there, just means being patient and looking online a lot for a suitable co2 canister that fits your needs as well as your budget.