Yamaha DTXpress/DTXplorer/DTXtreme

I think the part number is WK371500 (at least it is for the PCY135).  In theory you can order from http://www.yamaha24x7.com/ in the USA.  It might be worth trying to check the part number with Yamaha before ordering in case it is a different size for your cymbal. 

Or, try somewhere like http://www.rmcaudiodirect.com/Yamaha-Cymbal-Stopper-for-PCY135-and-PCY155-p/vu424400.htm

Keith