If you know what products you want then google will throw up good results and allow you to get the best price. Always ask for price match if your chosen supplier is more expensive on some items as many will do this.
If you need advice then find a local supplier or chemical specialist. You can usually find a good one by telephoning and asking for a bit of advice. If they are helpful but not pushy then chances are they'll be the same when you visit.
I would suggest listing all of the cleaning tasks you are likely to carry out and then discussing / listing recommended products. Ask for free samples. Try to use the smallest number possible, many chemicals can be used on multple tasks and the last thing you want is to be carrying too much and getting in a muddle.
I am in the Midlands so no good making any recommendations on local suppliers. The local supplier that I use is also our local Prochem authorised dealer, merely a coincidence as this was not why I selected them. If other Prochem suppliers are this good however I would definitely give them a go. I have checked out the same in your area and have copied the link below: -
http://www.prochem.co.uk/distributors/Greater%20Manchester.pdfRemember that local suppliers are usually more expensive but if you get a good one then it can be worth a little bit extra. That said you will often get a discount in store, but you have to ask.
Never rule out the likes of Wilko's or Home-Bargains. This doesn't portray the most professional of images but depending on where you are cleaning this may not be too important. We still use Stardrops Concentrate (as do the cleaners in our local hospital), VIM or Cif, Mr Sheen. These are also great places to buy Scrapers, Dusters, Dishcloths, Glade / Airwick Plug-ins, the list goes on. I would stay to proper suppliers for Scrims and Micrifibres (We favour Unger blue but SYR thick blue are half the cost and good quality / value). Again the message is 'SHOP AROUND'.
Good luck!