Nitrogen tanks in CA.
Can't rent them here. I spoke to three suppliers and a couple of those guys were amused at my request, which irks me. I rented oxygen and acetylene tanks up until about 10 years ago. But today I would have to buy a nitrogen tank. Bummer. Nitrogen is the only way I can figure to completely avoid the water vapor condensation problem. I don't want any water in my Avenger's air system.
I think that you're getting some confused info here. I've never heard of anyone having to buy a nitrogen tank. Everything that I've ever heard is as a lease. The reason is that they are a specialty item, that can't be refilled at every retail/commercial location. They are also pretty much too heavy (about 250 lbs.) for an individual to handle, and are typically delivered and exchanged at your location.
As for keeping water out of your system- A properly maintained separator and desiccant system on a compressor will keep it out. If you don't want to do your own compressor, there are quite a few places (more than having 6K psi N2) that fill SCBA tanks (dive shops, fire equipment supplier/service, etc.), and they are required to maintain the equipment to a -40 dew point, which means that you would never get water in your guns or tanks.
I've been using nitrogen for about 6 years now. Being a lifelong welder I had a few contacts. Normally here in NJ, they get anywhere from 6 to 7 hundred a year rental. Fills are 110. I pay 120 a year rental, and 110 for a fill. These bottles are 444 cu ft at 6K. They are specialty bottles and only keep a couple in stock. I normally have to call ahead, and give them 3 days. Most places won't rent to airgun shooters.
I bought tanks from Airgas, a one time cost there not very expensive. When I need a refill, they give me different tank that's already filled. That way the tank I have is never out of date, and I just pay for the gas.