Ductless MiniSplit Install Resources
There will be more problems than a nitrogen tank. You also need a vacuum pump to pump the line set down once everything is assembled and a vacuum gauge,  if the system pulls down and holds nitrogen is not needed but the non compressibles Need to be removed from the system. also depending on the length of the line set you will need additional r410 , I don't know any certified tech that would loan this equipment. Do you know there are HUGE fines for opening hvac/r equipment without a license, it has been many years but $20k rings a bell.  You would not have any warranty, better to find a tech that will do it on the side for cash if you do have it set up correctly. Also depending on your local electrical code , permits for the branch circuit. The hvac license is not a state thing it is governed by the EPA you can get the equipment on line but need a license to buy Freon.  For this reason I do not consider this a DIY job.