Bitfinex and Poloniex. Do it now, but be careful with BTC, it might become stuck now.
The price of bitcoin cash went surprisingly up and its good yhat you love taking risks and want to exchange your BTC for BCH, you can try many forums like cryptopia, bitfinex, allcoin, polonix for this purpose, but in my opinion i won't do it because i trust bitcoin more than any other currency and will not take the risk of exchanging bitcoin with any other digital unit