Answer to you problem is very trivial - ticket sale aren't started yet. Polish Railways (PKP-IC) starts ticket sale for this type of connections 30 days in advance, so as of today last day for which you can buy a train ticket is August 2 -->
Thus you'll just have to wait a bit and when the time comes visit official
PKP-IC website and purchase your ticket there. You can also wait until arrival to Poland, but then (depending on how in advance you'll be in Poland) some of the reservations might be already sold out.