https://www.amazon.com/SE-Bikes-Chain-Tensioner-GOLD/dp/B078GZ2475/ref=sr_1_10?keywords=chain+tensioner&qid=1554311796&s=gateway&sr=8-10 these will fit 3/8th axles.

Otherwise, these are some of the only 14mm tensioners that you can find. https://www.planetbmx.com/shop/blackops-14mm-double-bolt-alloy-chain-tensioner-black.html