The Closest Solder to Radio Shack Solder
There is nothing special about that solder.

It is 60% tin, 40% lead solder with RA (activated rosin) flux core that is 0.050&quot; (1.27mm) in diameter.
Any good quality solder that is close to those three details will be as good or better.
Personally, I prefer slightly more expensive 63/37 eutectic alloy with RA flux such a Kester 44, but there are plenty of other options such as Multicore.
If you live in a place that has restricted the use of lead-based solder they may be hard to find. Lead-free solders are a bit harder to work with and have a higher melting point.
Some imported solder may not be the pure alloy that you would expect from a brand name. If you buy brand-name solder from a proper electronics distributor you will not likely have any problems. Buying from dubious sources will increase your chances of disappointment enormously.