Not sure if it helps, but they are regularly sold here:
http://www.yodobashi.com/ペンタックス-PENTAX-レンズキャップ-Limited(リミテッド)-49mm-ブラック/pd/100000001000134343/
Japanese mega-camera-shop, where I always spend hours whenever I visit Tokyo.
So, they do exist...and can be bought apart. And are known to be available in Japan.
Five options, therefore: (i) plan a vacation to Tokyo soon, and buy it yourself, (ii) make friends with someone in Japan and have him/her ship one to you, or (iii) if you can't afford to buy any friends, and if I remember this for next time I go east, then I'll gladly schlep one to Europe and post your way, or (iv) figure out if Yodobashi will ship to you.....or (v) keep looking