First, you don't want to buy a tank online, the shipping is too high.
Second, I've read many times of folks getting parts from several places and then spending a lot of time trying to get the right connections between the parts. More connections, more chance for leaks.
The All-in-one types are so simple. ~$90 online +shipping and you have everything; dual guage regulator, soleniod, bubble counter, needle valve, all hooked together and ready to go. Add tubing and a reactor of your choice.
You can get a JBJ set locally from Pet City if you don't want to order on the internet. There's lots of places to get CO2, tons of welding shops here. I paid $75 for a 10 lb filled at Aeriform, on the NW side of Houston. Check home brew shops also for tanks and refills.
If you can fit the 10 lb, why get half the volume to save $10? Not worth the trouble to drive to refill twice as often.