I went to Ace Hardware to find rubber friction-fit caps that @eatmykudos mentioned, but with no luck. Then I thought I'd make some by myself using heat-shrink tubing (for electrical wiring). I bought some 21 mm (when flat) heat-shrink and cut them into 1 inch segments. I inserted a house cue half way into the cut tubing and gently shrunk the top half with a lighter. I made 5 in like 3 minutes. It's a lot cheaper this way, and you can make it custom fit on your shaft. Be sure not to over-shrink the section of tubing on the ferrule of the house cue, as it may become a bit too tight to pull back out.
The whole DIY cost less than a 2 bucks, including the lighter. Hope this helps!