Where Can I Find a Female Hexagonal Plug?

The more usual term for this is a pipe cap. A pipe fittings or hydraulics supplier would be the natural source.

Interesting...

When I read "Female Hexagonal Plug 3/8" bsp threaded" I think of a hex-drive plug, thus:

I was sort of ??? by the phrasing. EN 10242 is not a standard for brass or bronze fittings, I think. Perhaps a cap is correct.

I use these in NPT, but have no idea where to get one BSP.

Never bothered to check the difference in NPT and BSP. Will they interchange? If so, and this is the correct type of item, I have a lead for original poster (if brass is acceptable).

You might be right; the description can be read in more than one way. Hoping for clarification....

I'm with Tornado on this one. For me Female plug is an oxymoron.

I think OP is referring to a cap as depicted in the bottom picture.

So far I have found NPT (National Pipe Thread) pipe caps (female threads) in brass, with an outer hex profile; and hex socket plugs (male threads). No luck yet with BSP and bronze. Both are in McMaster-Carr Catalog 116 (Web-searchable).

Female dictates threads on the inside of the item.

Thus cap, as Tornado said. Kaisan has supplied the correct picture.

That is, if the OP did a proper job of defining the item in the first place. Which is always a crap shoot.

BSP= British Standard Pipe check this page out, may help? http://hisg.50webs.com/Basics5/basics5.htm

As far as a vendor for the part. As mentioned above, McMaster Carr, or Grainger, or Fastenal???