What rob said. Sheldon Brown’s site has a good explanation.
http://www.sheldonbrown.com/chainline.html
If you’re just going to be casually riding around you might not need to bother respacing the axle or redishing the wheel.
And you might also find that by moving the chainring at the front from one side of the tabs to the other that you get near enough chainline not to need to bother.
As for parts – you need a single speed freewheel. Charlie the Bike Monger has a good selection, from the ACS (perfectly fine but makes a stupid noise after a while), the Shimano DX (better sealed, eventually wear out), and White Industries (gold standard, rebuildable, indestructible but pricey).
You can often find them (and similar) on ebay.
Or, just buy a cheap second hand wheel with a modern freehub on the classifieds 😉