Which spring?? Which pistol??
Try your local gun smith.
It depends on the pistol, but VERY generally speaking, you clear the gun, take the slide off (the slide removal process depends on what type of pistol), take the guide rod out, take the barrel out, then put cleaning solvent on the parts, let it sit for a little bit, wipe the solvent off, put on a very thin coat of lubricant on most of the metal parts, then put the gun back together.
It depends on the gun. Every manufacturer is different. If you can't find it through a Google search, contact the manufacturer. Or, just bring it to a competent gunsmith.
Get two pistols and put them together and it will make a double pistol! TADA!
Most are not made to be taken apart to begin with.
Gun shows, gun shops, e-gunparts, or you might just type a query in a search engine for it. Just put the make and model of your pistol and "magazine", and see what turns up.
It depends on which gun. Different guns assemble in different ways.
with a hand pump
Check YouTube for Marksman Repeater, I have found several that show you how to take it apart and put it back together
Gun shop or gunsmith
Best left to a professional.