Rohm is pretty good, have heard Dowex is also good. The problem with Rohm is that they are having production problems so you will see a lot of suppliers run out of it.
I've tried Purolite and Rohm and found Rohm a bit better. I now use another brand which I am very pleased with the results but I am the only supplier of it in the UK, the downside I only supply to Scotland.
There is also another brand which most suppliers are changing to called Bayer.
These are the brands on the market,
Doug