I love Connecting Threads and Superior threads. Both have some great deals and options.
As for cotton vs polyester - I use a variety including silk and metallics. It all depends on the project, what I'm looking for in color & texture and the technique. So needless to say "It's time to play". As much as we all enjoy the search for the perfect fabric, all of the other things we do to make these gorgeous quilts are equally important and an adventure to enjoy. So step outside your comfort zone and try some of the other threads.