You can purchase BLOCKTIX from these places
Go to Bittrex
Bittrex is one of the larger crypto to crypto exchanges offering a large number of trading pairs into bitcoin - including all the major cryptos such as darkcoin, nextcoin and litecoin
Go to HitBTC
The most advanced cryptocurrency exchange to buy and sell Bitcoin, Ethereum, Litecoin, Monero, ZCash, DigitalNote, Ardor.