I get swap refills at the welding shop. Sure the bottles are ugly but I’ve never had to pay for a hydrotest.
I recommend getting the biggest bottle you can. I pay $20 to fill a 50 pound tank delivered to my office. Once my 20 pound tank goes dry(been 2 years now) I’ll hook up the big one and probably die before it does. 20 pounds costs $12, 5 pounds costs $10 and my 2 pound traveling bottle costs $8.
Build a relationship with them and it pays off. The welding shop didn’t have a 5 pound tank swap one day so they gave me another 20 pounder. Somebody left a 2 pounder there and never came back for it so they gave it to me since there’s no market for a bottle that small
That’s awesome corky. The guys at the new welding place are getting to know me, they only opened in the last year or so. I don’t get in there too frequently since the tanks last so long, but they know I’m a brewer and we talk beer. I need to take them some samples next time. 
Our company probably spends $50-75K annually with these guys. But they’re afraid of drinking my homebrew, one of the employees lives down the street from me and visited once but since he’s a hard core Silver Bullet drinker my stout overwhelmed him after one sip.
Maybe you can serve them a glass of water with a drop or two of your stout added
The guys at my local place like good beer, they like to go to the local bottle shop/pub or one of the breweries after work.
FYI as long as there is any liquid CO2 in the tank, the tank pressure will not change. Once the gauge pressure begins to drop, you’re “running on fumes”. It may still last awhile, but there is NOT a gradual drop in tank pressure as CO2 is consumed from a full tank.