Sorry, but the minimum quantity I sell them via mail order is 50 (for $42.50 shipping included). It's just not worth my time to pack and ship smaller quantities of them. I do sell them singly at the shows we attend, however, since all I have to do is to hand them over the table to someone.
As for making them, you could start out like I did, using a drill press with a moving vise. I used a 1/2 roughing milling bit to cut the slot laterally across the top of the cut by placing the PVC endcap in the vise and moving it towards the cutting bit. The cut is not very smooth, however, but it made a better overall cut. I first tried to cut across the top, but I had a lot of problems with the endcap chipping.
<img src ="http://www.cornsnakes.com/pics/drillpress01.jpg">
Of course I could only do one at a time, and the thought of making enough of them to sell was out of the question.
I had so many people asking about them, that I felt I was justified to spending the money to buy my own milling machine to be able to make them somewhat quicker. A friend of mine made up a holding jig so I can cut 6 of them in one pass. The mill has a motorized table, so it just passes the caps under the ball end milling bit and it cuts the slots into each cap in succession. When I get time, I may make a longer jig so I can cut more of them per single pass.
<img src="http://www.cornsnakes.com/pics/mill04.jpg">
Once I have cut enough slots in the caps, I flip them over and using another bit, will flush off the bottom to make them perfectly flat. I didn't do this when I was using the drill press simply because there was no easy way to do it that way. I also debur the edges of the slot cut to remove the sharp edge that remains after the mill cut, and they are then good to go.
As you can easily see, you are paying all for labor for those water dishes. Each one requires two cut passes with the mill, and then each one has to be handled to take off the sharp edges. If I could have bought them for the price I am charging when I first needed them, I would have done it in a heartbeat.
Right now I am trying to get enough of them cut up to last me through the shows this season, where I expect I'll sell a fair number of them. Once the babies hatch out, there is no way I'm going to have time to cut up more of them. So when these run out, that's it until next Winter.