Recertifying nitrous bottles?

I don't understand why a welding supply could not take care of this. Here in town, there are two different companies, and at least one handles all sorts of "bottled gas" besides argon, CO2, and oxy-acetylene. I don't think they hydro-test on site. I think they are sent off to the "big plant" where ever that is

In the case of smaller bottles, I would not be suprised the hydrotest is nearly as costly as new bottles