I use wire solder. It is convenient to handle and easy to tin a soldering iron with as well as adding an additional touch of solder to a joint when needed.
I generally flatten a couple inches off the roll of the solder wire, no matter the thickness of it, to a skimpy near paper thin look.
Makes for even quicker melt and flow when tinning or doing that small additional touch to a finished joint.
Simply hammer it out on the anvil portion of the vise as needed (keep it clean when doing so)
Block or bar solder was/is generally used to tin those large copper headed soldering irons used in radiator and sheet metal production work. The bar was much easier to use to (re)tin the soldering copper as it simply sat in place on the bench by it's weight and doesn't move as the soldering head is run accross it.
I've always used common plumbers paste flux in gun work of all types. Available from Home Depot or similar stores. Never a problem as long as everything is clean to start with and you don't over heat things burning off the flux which will destroy any soldering attempt no matter the solder & flux used.
I stay away from using 'acid flux' (Zinc chloride) now. It works great but can cause real bad after rust problems if not rinsed from the surface completely,,something not as simple as it would seem.
You can make your own by dissolving as much zinc in muriadic acid (hydrochloric acid) as it will take. Smokes and fumes so do it outside!
I used to to take dead flashlite batterys and strip them for the casings which were (still are??) zinc and make the stuff that way.
The black compound inside the case was manganese dioxide. Used to keep that and add about 5 to 10% by weight to the Nitre Blue salts to bring the melting point down for a lower degree temper bath sometimes needed.
My days of Mr Wizard @ Night are fewer now and it's been quite a while since I've ripped any EverReadys apart for their scrap components,,but it shows what could be done,,at least at one time.