Right now I have multiple co2 set ups and they came from all over.
I bought all of my co2 tanks used from other aquarist here on the islands.
1 co2 regulator was bought at Aquascapes, the rest was purchased by quitting aquarist on Plantedtank. All my diffusers came from GLA. And I don't use a drop checker, I go by eye.
I use the 5lbs tanks atm and they last 2-6 months. I depends really on how much I am diffusing. One tank I have outside in Direct sunlight that stays on 24/7, no fishes, at about 5/6 bps. My tank last normally 2 months. And other tanks that is on for 6 hrs a day, at 2/3 bps that lasts me 6 months. Sometimes longer.
Another big factor is the way you are diffusing your co2. I recently switched to a inline diffuser and I couldn't be happier. I use a lot less co2, judging by my BPS, but get a lot more co2 diffusion into my tank thus having to fill less. Also with a inline diffuser, I don't see extra things in my tank like a Glass diffuser,suction cups, tubing, and EVEN bubbles. Everything is connected to my canister filter. I don't know how long my tank will last now. Maybe a year.
I tank exchange at Gaspro for around $25 for a 5# tank.
I know you already know this but be sure you have sufficient lighting to use a co2 setup. Atleast 3wpg+ or it would be next to pointless to have a high tech co2 setup. I don't want you wasting money on something you don't need.
And please do keep in mind. I might not be right in everything I said but that's just what worked for me until this day.