Hello everyone!
English is not my native language, I use a translator, so I apologize in advance for inaccurate wording or terms. I will be glad if I am corrected if necessary.
I have been reading various topics here for quite a long time, I think I have figured it out enough to ask the right questions, or even help someone.
I chose the N2 method (SCBA or SCUBA) for myself. I live in Eastern Europe. I started by looking for a cylinder and N2 <-> AIR adapters. In my country, there are at least 3 suppliers of cylinder gas, I wrote a request to each of them. So far, I have received complete information only from one, but nuances have already appeared. This gas supplier seems to be a German company, and I think they can be trusted. They claim that all their gas is at least 99.995% pure. At the same time, there are also options of 99.999% and 99.9999%. But cylinders come with different types of connections: DIN 477-9 or DIN 477-10, depending on the cylinder volume and gas type. And it is necessary to properly choose the adapter N2 <-> AIR, according to the connection on the cylinder, to connect SCBA or SCUBA equipment. It will be either DIN 477-9 G 3/4 <-> G 5/8, or DIN 477-10 W24.32x1/14 <-> G 5/8. At the moment, both adapters are available for ordering online in EU, and I ordered both. By the way, in the description of the DIN 477-9 G 3/4 adapter it is written that it is for oxygen, probably this connection is "originally" used for O2.
DIN 477-9 G 3/4 – 5 l (99.995%), 10 l (99.995%) cylinders.
DIN 477-10 W24.32x1/14 - 8 l (99.995%), 10 l (99.999% and 99.9999%) cylinders.
I hope this will be useful to someone. If so, I will describe the further stages of the installation later.