Like bubba says, Precision Rifle in Manitoba makes a .25 ACP breech plug in both Huntsman/Sidekick sizes. (5/8" and 7/8").
I have the breech plug for my Huntsman and would reccommend it to anyone that wants almost no blowback and easy finger and thumb removal of spent primed cases.
If you make a 209 plug without carriers you will need a tool to pry out spent primers.
The .25 ACP brass and small rifle primers are actually both cheaper than say Knights Full Plastic Jackets and 209 primers for comparison. If you buy the brass in quantities of 500 (not from PR but a online sporting goods dealer like Midway), they are way cheaper than Knights FPJ's and you can only buy the FPJ's in 100's. The brass can also be de-primed cleaned and reprimed if you want to deal with that. A good media tumbler might be in order for cleaning the brass, as I haven't found a good alternative yet. For the price, I just discard them for now.
I would imagine a Tool and Die Shop might charge you a bit for tooling up to make one plug, unless you know someone that does it personally.
Direct link:
http://www.prbullet.com/nef.htm