If you are looking purely for an online supplier then I haven't found anyone who is cheaper once you factor in delivery. Mind you I am not the worlds best with search engines lol. Delivery is the biggest problem mainly due to the length. If your not wanting to shop about locally then you have already restricted yourself, although having said that I don't have a local supplier for aluminium myself.
Other thing to consider is the amount you will be spending with the company, Jonathan probably gets a better discount than some because of the amount he spends with them, when buying as a business you will be able to build up a volume discount.