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.
Go to EtherDelta