"Mick-VA"Another method I have used in the past was a 90cubic inch 4.5k psi Air Venturi or Ninja tank that has the regulated output. The regulated output is supposed to be set at 3k psi but they really only put out between 2700 to 2800 psi. So, depends on which gun you tether and it's performance at that range.
I was initially aggravated with it til I figured out that it filled my Prod to the exact fill it preferred at 180 bar, so I kept it. I then began using it for tethering unregulated guns for bench rest shooting. Lotta shots! They seem to go for $299 but I see them for sale used quite often for much cheaper. A lot of unregulated guns like to shoot at 180 bar, right in the plateau of their curve. Also, most regulated guns have set points of 150 bar and below so this works in that scenario as well. They can also be purchased at different "set points" for regulated output. The dedicated outboard Regulators are nice but this works well in a pinch. Air tanks plus seems to have them with set outputs of 2400, 2600, 2800, and 3000psi. And if you already have a small tank with paintball sized threads, you can just buy the regulator for $98.