Here is a link to the set (I think) I bought. They are also good for geocaching I am told. They are super strong too so I'm not worried if I throw them into a bag fill of locks or wherever. I weigh 185 and can staind on one foot on top of one and it doesn't flex. Pretty cool! P.S. - Sorry I couldn't get the hyperlink to work. :(

Edit: the hyperlink does work!

Soda Bottle Preforms and Caps (30/pk) https://www.amazon.com/dp/B008MB1QNY/ref=cm_sw_r_other_apa_i_y3JIEbG9Y95C9