Where can I easily find them?

Would these work?

http://www.oreillyauto.com/site/c/detail/DAG0/090046.oap?ck=Search_090046_-1_1157&keyword=090046

yes

aluminum works, but i have found over the years working with aluminum recessed hex plugs, that all hex wrenches are not created equal. and when the wrench doesn't fit EXACT, the hex strips, or rounds out, causing me lots of grief removing them after the fact. i prefer steel or brass for just this reason alone. stainless plugs are good, but you have to make sure the threads are coated with something that will prevent galling. if you don't, same grief applys. just my experience and preference.

All good points. And the port must be drilled and taped. So why go to all that trouble when you can just tap in the core plug? Pipe plugs are a lot more trouble to install, can be a problem to remove, and do not any better job.

What about putting the restrictor in the front clutch port of the valve body instead?

Put it in the case It's much easier . Grease the tap to catch the chips.

Putting the screw in restrictor in the case is pretty easy.
I had a stainless steel plug I was going to use, but my drill bits did not want to drill it. The aluminum drills easy, and the plug does not need to be installed tightly, so it is easy to remove. Not to mention it will be covered in trans fluid. The steel core plug is what comes in the Transgo shift kits, but if you want to experiment with different orifice sizes, the screw in plug is easier to remove, drill, and re-install. Brass would be easy to drill also, and might be able to find in the plumbing section at the local hardware store?

If I went the core plug route, what do you recommend tapping it in with to tap it in even? Do I drill the 3/32 hole before or after the core plug is in place?

i would drill the hole first. reason being it's possible to get a chip or two behind the core plug you would not be able to get out. then it's possible they would migrate at some point to places you would not want them to be, causing problems. to tap it in even, i would use a punch of the appropriate size to get it almost flush, then finish it off with a flat piece of material that would lightly contact the surface of the case. and a side note to those that think brass is "easy" to drill. while brass is not really difficult to drill, the problem lies in the "breakthru" of the bit. at that time, it is super easy to have the bit "catch", and when doing so, the bit, because it is on the small side, will almost always break. another caveat with hardware store brass plugs is the alloy used can vary from super hard/VERY difficult to drill, to super soft/very gummy, also difficult to drill. this is just my 42 years as a machinist working with almost every material except titanium.

I use a small 1/4 drive socket that fits inside the plug on a short extension to tap them in. That way I don't dimple the tranny case. It doesn't matter how deep you go with it as long as it is past flush with the case. And it doesn't have to fit very tight because it is essentially trapped and cannot go anywhere.

I drill them after they are in place. It's just easier. Grease the drill bit to catch as many chips as possible and then use the straw to spray brake clean or carb cleaner or the like above the plug to wash out any surviving chips.

Have done quite a few that way and have never had a problem.

Many thanks to moparx who sent me a 1/8 NPT tap and several hex head plugs at no charge!, I'm much improved and nearly overlap free. There is still a slight bit. I used a 5/64 (0.078125) drill bit for the hole. I would really like to get rid of all the overlap, but I'm just not sure that's possible without changing out the 5.0 lever. I guess a slight bit of overlap is ok?