I tried to buy from https://www.buy-zcash.com/ but the application does not seem to work: the buy button is never activated even if i fill in a valid zcash wallet address
Try trading on exchanges for ZEC. Some of the exchanges are bitmex, poloniex, kraken, etc. Most exchanges will allow you to buy zcash in exchange for bitcoins.
ZCash is now listed on CoinSwitch. You can purchase it from https://www.coinswitch.co/exchange/btc/zec/0.1
CoinSwitch is a cryptocurrency and altcoin exchange aggregator. They have integrated many leading exchanges across the globe like ShapeShift, Changelly, Bittrex, Cryptopia to provide best exchange rates to their customers. Currently, they support over 100 cryptocurrencies and more than 4500 pairs to exchange.
If you already have bitcoins, you can use for example ShapeShift or the Jaxx wallet to buy ZEC.
Switchain is a good tool for this kind of exchange. It's better than shapeshift since they have their own trade pairs but also compare rates to other exchanges(including shapeshift). try it out.