A-1 Auto Parts in Mebane (Alamance County)
Just setup my first co2 system. I have a 100 gal tank and live a fair ways from Durham, and the fire extinguisher place only seems to deal with 5# cylinders for fill-while-you-wait. I only make it to Triangle Tropical once a year or so and think I'd need to fill a 5# tank more often than that. But they were very nice and responsive to my email, and if I lived closer, I'd go with them for sure.
Anyway, I live near Mebane, and A-1 Auto Parts sells 20# cylinders (only). It is initially pretty expensive, but the convenience for me was the key. I payed around $180 for a full tank. They swap out when needed for another full 20 pounder for around $35. I can return the cylinder at any time for a refund of around $135/140 (don't have the receipt and agreement with me right now, but that's the ballpark). They have Saturday hours - a huge plus for me since I work long hours during the week.
Thought I'd post in case it helped someone else who lives out this way.