Hi,
Squires are a renowned cake decorating supplier. Not only do they have an extensive online shop but they have a shop where you can look around in person. I don't know where you are based but if you are in London they are about 50 minutes away on the train in Farnham, Surrey. They also run icing / decorating classes too.
Their website is www.squires-group.co.uk
Another supplier I use is www.cakescookiesandcraftsshop.co.uk
They have a good range of products and delivery is always prompt.
With regard to your question about flower petals, it is much cheaper to make them yourself but it depends how much time you want to spend. I bought a lovely set of four blossom plunger cutters from Cakes, Cookies and Crafts and made them from rolled out sugarpaste which I had coloured. You can make a lot in one go and then store them in an airtight container once they have dried out.
I find that the cupcake cases are probably the most expensive item - maybe that's because I like the pretty ones. If you google for muffin cases or cupcake cases (because I find that regular fairy cake cases are too small) there are some suppliers who sell packs of 1000 which work out much cheaper than the smaller packs.
Good luck with your new venture!