I can answer the first part. You get nitrogen from a welding supply place. For small bottles, you normally buy the bottle (tank) and pay for the fill. Usually you get a used tank for your first purchase and just exchange with a used full one when needed. You need a regulator to get from the high pressure of the bottle to somethign reasonable. The cleapest way to do this is buy a regulator for MIG welding with a flow meter. Make sure it is for Argon and argon mixes and not CO2 (which uses a different fitting at the tank.) You can buy a cheap end on off ebay, or get a USA made Smith for about $60. The low pressure connection is some sort of a weird fitting, I bought mine at the Local Airgas dealer for $15 and you can put any kind of hose on it you want.
20 or 40 cu ft is about the smallest tank, and is usually in stock because of HVAC and plumbing guys who haul them up on rooftops and such. This is the exact same cylinder use in the portabel oxyactelyene outfits at home depot. Harbor freight has the tank (with an oxygen valve) for $99, and has 15% cupons all overe the net. Ask you local weldign supply if you can trade a 20 cu ft oxygen for a nitrogen cylinder if there price is over $85. Mine lets me do it all the time. BTW, the HF cylinder is USA made.
BTW, 20 cu ft, means 20 cubic foot at atmospheric pressure. the cylinder is about 6" in dia and weights 15 lbs.
I won't comment on purging, but there are two strageties for purging in general : Flood with 3-10x the volume needed to wash the moisture laden air out (dilution is the solution to polution), or pull a vacuum and then refill with nitrogen.
If you are dealing with you local welding supply for more then a one time purchase, apply for an account, they treat you better, and you can rent a tank when needed. I have acocunts with 3-4 places, and used to be more then that before there were so many buyouts. There is almost a perceptable mistrust of those without accounts, as if you are filling stolen cylinders.
You may be able to get small N2 tanks at local HVAC supply places as well. I stick with the welding places for safety.