If you don't have any luck purchasing end caps you could try making your own. Long time SC member Fung Pang came up with a great way to home brew endcaps. You might have to Rube Goldberg some of the pieces, but it should work.
This was posted some time ago, but I am reposting it now for anyone who hasn't seen it...
Fung Pang uses a Dremel Mototool in a drill press stand and a few bits. These are pretty basic tools you might want to get anyway. It would cost about what you would pay to have the work done for you and you'd have the satisfaction of learning a new skill that will come in handy.
Here is a slightly expanded writeup on FP's method that might help.
"How do I make endcaps for my WTC without a lathe?" is a perennial question for new sub guys. Either they don't want to buy a set from the usual vendors or they have an odd size tube for which there is no standard endcap available.
He cuts a length of the WTC tubing to act as a mold (he puts brass tubing over the tang of the bits to act as a spacer.)
Note - when he took the photos he didn't have his usual Dremel bits (#199, #198) in the chuck - so he stuck one of his others in there, but you get the idea.
Here are his instructions.
How to make an odd size end cap
Why an odd size? You could buy a standard one, why make it?
I need a cheap WTC to go in my cheap sub! That means I have no lathe! Not everyone knows how to use one - I know I don't!
I use part of the pipe as a mold to cast an endcap, so any size pipe can be used.
For how to make a mold and casting, goto "smooth-on's website" but remember, most castings shrink; not too much on the small pipe, but the big pipe is a bit on the loose side.
Another thing to remember - the inside of most PVC pipe is not round.
All these end caps are made for pvc pipe you find cheaply in a plumbing store. And it could be any type of pipe.
Small one is 2", the big one 3".
The taps (ears) are to make removing the end cap easy (I copied this from another Subcommittee member's idea )
Making the mold...
One ring goes on top of the other.
Outside ring made from card stock, about ½ inch tall,
center ring is made from pvc pipe end cap, with cut out for taps (ears), about ½ inch tall
note: if you don't need taps on the end cap, the center ring is not needed.
Inside ring the pvc pipe is about 3/8 inch tall.
Cutting the end off a PVC pipe cap with saw in Dremel drill press
This is a setup on a dremel drill press, a real drill press will do even better with lower rpm and more torque.
I set this up to take a picture, but it is a bit misleading.
The cutting bit should move in to the chuck little more, the spacer should be on the cutter.
Think of it as a router table, I start up on the Dremel router table setup with the stopper, it works very good on the end cap, until I try the taped end cap!
This doesn't show the pins that go on the baseplate of the drill press that you push the endcap up against to get an even depth cut.
I use the drill press as a upside down router, use brass tubing on the cutter as a spacer to adjust how deep it cuts and move the drill up or down to cut the width of the O ring, or just as Deep Sea Designs shows at http://www.deepseadesigns.net/wtcplans.html
Cutters - Dremel #199, #198 and spacers [brass tubing] to cut the grove for the 'O' ring
The brass tubing goes on to the shaft of the cutter, one after anther to build it up, so it will not cut too deep, or the Dremel will jam to a stop. I like to run my Dremel at low speed if I can, I don't want to smoke it.
The cutter is thinner than most of the “O” rings that I find.
To fix this set the first cut at the high point of the cut, turn the end cap as the cutter cut in to it, when finish cutting around the end cap.
Put some paper under the end cap, this will adjust the end cap higher for next cut, check for size.
Add paper, cut, check, add paper, cut, check, remove brass tubing., cut remove paper cut, check, remove paper cut check.
Put a vacuum next to the cutter so it sucks up all the PVC cut out of the groove to stop the indoor snow storm!
Here is a view of the jig used for cutting the groove.
Thanks FP for the great idea!