Do a search on 'axles' and you'll get enough stuff to read for a month or so!
We paid $750 each for new complete axles with hubs and electric brakes last year out here on the west coast - check around and you'll find other dealers nearer to you...we installed em' ourselves - bolted right into place without any problems...
Don't forget the tires - if your trailer has been unused for any length of time, look into getting new tires, even though yours still look good and have tread - trailer tires 'rot' while sitting around, and if you blow oneout on the road, you can damage you trailer, water tank, belly skin, etc and will cost big bucks to fix - new tires are way cheaper!
Propane copper tubing is nothing special - get it from Home Depot, Lowes type big box hardware stores...
Good luck on your new 'toy'...