I got mine at thinkauto.com
No need to drain oil as the gallery should only be full when the engine is running.

One thing to watch out for: screwing a bulky sender/t-piece directly into the same hole used by the original oil pressure switch can be a bad idea.

I have heard of the head cracking or even snapping under the additional weight and vibration.

I mounted my new sender remotely by buying a male-male adapter to fit into the original oil switch hole and a length of braided hose with female fittings on each end.

That way you can mount the t-piece at a suitable position in the engine bay and use the hose to connect it to the oil gallery/head.

HTH