you are not going to find them stateside. it's more common sizing in europe, but not used here. a G1/4 tap runs bout 75$ special order.
closest you will get here is 1/4NPT which is not a parallel thread but a tapered thread that seals by interference between the threads. those taps run about 5$ at any hardware store, or are included in most automotive tap sets. a 1/4npt fitting will thread about halfway into a virgin G1/4 port and seal with some tape on the threads. if you run a 1/4NPT tap down the bore first, it will thread in 75-85% of the way normally. this size is common for sateside plumbing applications, so you can find brass fittings just about anywhere.
as for adding a port to a radiator, assuming you have a tap of your choice, you could use a flanged fitting or get brass/copper round stock from a metal supply place (such as McMaster-Carr). measure the OD, carefully cut/drill that size OD into your radiator end tank. then silver solder the fitting/bung into place. if you aren't comfortable with using a torch and solder (this is generally higher and much more heat than electrical soldering), find a local automotive restoration shop and ask who they use for radiator repairs. those guys replace cores and end tanks all the time and would be the ones to approach about having it done. don't expect it to come cheap.
you are probably best off adding a splitter/T-Fitting just before or after leaving the radiator.
also for what its worth, my biggest bitch with the watercooling PC industry is the damn G1/4 standard.
only word of warning I can add is that this stuff tend to be a lot thinner and cheaper than you'd expect for the premium they charge for it.
If you have access to a lathe, I'd take a 1/4npt to 3/8 pipe flare fitting, part the flare portion off, keeping the flange. Drill and tap 1/4NPT into the radiator, then thread in the fitting and solder. Then you could tap for G14, assuming you got a hold of a tap. The flanged surface should provide enough landing for a sealing o-ring of a standard G1/4 fitting.