The 25acp breechplug is available from
www.prbullet.com (scroll down left column and click on NEF ACP Breechplug)
I have this set-up for my stainless sidekick and it is ok. I use a Remington 7 1/2 small rifle primer in a 25acp brass case and the primed case is inserted into the breechplug after loading the muzzleloader. The pr (precision rifle) website suggests using Triple 7 ffg powder and that is what I use. This system works fine as far as I can tell, as I never used the NEF supplied breech plug from new so I can't compare it. My only complaint is that Triple 7 is a dirty crud ring causing powder and I clean mine after 2-3 shots. After 3 shots it is real difficult/ damn near impossible to seat another bullet, is what I found out. I use the 40 cal 200 grain bullet and sabot from pr as well. I take the plug out and soak it in pr black powder cleaner and then use the pr black powder cleaner and patches to soak the barrel and it comes clean fairly easily. The plug however takes some work as it gets kinda sorta wore in/baked on if you don't keep at it. I just detest having to clean after so few shots and no I won't spit patch cause it just pushes the crud deeper in and against the breechplug as I have experimented with this as well. I don't use it often but when I do it is fairly accurate and works ok.
If I had to do it over again I would buy a black powder rifle that could use Blackhorn 209 and the 209 primer simply because of the cleaning issue. I'm not much of a muzzleloader shooter, just own one to extend my deer hunting seasons. I do most of my shooting with Topper 30-30 rifles, and a .223 Ultra, now that is fun.