I thought I would get a magnetic plug for my 4 speed after we're done rebuilding it, but I'm not finding one. Do they exist, and if so, where?

I just drilled a blind hole in the plug and epoxied in a neodymium magnet in it.

Sometimes that's the easy way rather than searching aftermarket.

Hemi 4-speeds had them from the factory.

None of them that I used did I raced them from pre mid year 1969 model years though
I worked for the local telephone company back then and I would remove the magnets from the bad ringers, usually the coils would go open, and epoxy them into the drain plugs
I've seen some of the later aluminum case O.D. 4 speed tranny with a magnet fastened, glued, inside the cases on the bottom also
OP, if your dealing with a stock cast steel case slide it in near the drain plug and use it that way

The A12 Hemi one I just rebuilt had one it it also.

1/2"-14 NPT magnetic drain plug, available from just about everywhere.

Would it be beneficial to buy or make one like Pac said that had more magnet protrusion?

surface area is bound to get more items, but most rare earth magnets are also very brittle.
so you have to take that into account.
plus it would be based on flow past it as to how effective it might be.

will this work? https://www.ebay.com/itm/ONE-Muncie-T10-...?epid=660741957

Noted

Ok,I should have said most hemi 4-speeds I have had or repaired ( more than 50) had a magnet, the factory part had a 5/16 dia by 3/4 long past the pipe plug end, sticking out in the oil? piece of steel that was magnatized, not a ceramic or rare earth magnet. If you use rare earth magnets go with samarian cobalt, more stable in high temp applications.

That's the fill plug. Drain is smaller. I would guess 3/8" NPT.

Well, ya made me look and you're right...1/2" fill, 3/8" drain.