I need a pre-assembled pill, which as I understand it, is the brass housing including the led driver board and the LED itself?
I had ordered a Convoy S2+ host, to experiment with. I also have a fully assembled Convoy s2+, and indeed I can swap the pill back and forth between the units.
However, I don’t want to create my own pill assembly. So, my question, where can I order a 4500k Nichia 519A pre-assembled pill?
Unless someone has the parts to part with (and a driver that you want) then I think you’ll have to ask Simon if he’ll assemble and sell one to you (good chance that he will).
Slight chance that you could buy the pill (but likely not with the 519A) from Mtn Electronics, Mohr Lumens, or JC Customs…would need to get in touch with each as I don’t think any of them list is as a separate item on their websites. They all sell the hosts and/or lights so maybe they’d have one and solder it up.
Neither of the ones I recently got from Kaidomain will work in the S2+ hosts I have although at first glance they look as if they might.
The brass pill is just like you described….holds the driver on one side (which, depending on the driver, may or may not have a retaining ring to hold it down, and/or solder), then the other side with the shelf holds the copper mcpcb which the emitter is mounted to.
Last order I placed, I asked Simon if I could buy a spare pill and he included it for free. I think he’s done that a lot but won’t list it as a part for sale for whatever reason. Came with two retaining rings, too…sweet.
A small, but maybe somewhat essential, remark. Convoy changed the threads of the S2+ a few years ago. If you bought yours recently from Simon himself, you can assume your light has the new threads.
I used to buy pills from Fasttech but now they are all out and I switched to Kaidomain. The last ones I bought there still had the old treads. I don’t know any other reputable seller that sells them (old or new).
Your question looks like economical use of resources. One light with interchangeable pills. But atm I would advise to buy a second light, or a host, from Simon. And then swap, or put in, the emitter of your desire. I’ll bet the extra costs are beareable.
How often you plan to swap the pill. It takes more than just a few seonds, and it is not something I would like to do if the circumstances were not optimal.
FYI: reporters do have camera’s with interchangeable lenses, but in real life each has its own camera body because swapping lenses is too cumbersome.
the host should have come with a pill, so it’s just a matter of assembling it with a driver and MCPCB with LED. (and some thermal paste to conduct the heat of the LED to the pill)
When you’re not happy doing it yourself, Simon (owner of the official convoy store on AliExpress) is the one to contact.
Unless you live nearby, i am willing to help you out.
Okay, well being new to terminology, I bought a host and now don’t have a way to fill it, haha. I don’t intend to swap. It was only $12 so its okay. I just thought I could buy internals later, but Simon doesn’t sell them. So, I don’t need a pill, I need a jelly donut. 
As Yokiamy already wrote: if you have a host, you probably have a pill. You just need a 16mm led+leadboard and a 17mm driver. And a soldering iron, of course.
I probably have a pill but not sure you will like thr driver and it’s older style so the threads might be different.
Simon sells all needed parts!