I've only ever used one 9" diamond disc nickk it being a Bosch and it went through engineering bricks easily enough kicking up a dense cloud of dust. Prices on eBay from around £12 upwards. Good luck.
norton are the best but it depends on what quality you buy they probably do one premium as well and not top quality to compete with the crap ones made by others . you get what you pay for
Do you have any specific cutting requirements? Or just a general purpose one?
The reason I ask is that if you are only cutting the same thing all the time, you can maximise the disc's life by buying one suitable for that material. A hard material (like granite) is better cut using a soft disc and a soft, abrasive material (like asphalt) is better cut using a hard disc.
If you are searching, put both 9" and 225mm in your search as some people do not list in both sets of units.