I have seen posts about premixed glazes quite a bit lately. Perhaps there were always questions, but I am noticing them more. Linda my suggestion is to go and get Tom's book and then buy the dry materials and then mix your own. It is really pretty easy once you get the steps down, and there are many videos on how people mix their own glazes. If you are going to buy premixed dry glaze you will need to buy buckets, a respirator and a sieve. To mix your own the only other tool you need is a scale, and many already have that.
The cost will be about 10% what you would pay for dry mixed glazes. I say give it a try and test. Then after a few glaze batches you can experiment with a base glaze of your own and then the real fun begins.