Decide what tyre you want and then check prices. You have to do your homework, don't just assume that a local dealer will be more expensive and likewise, don't assume that buying them on-line and them having them fitted locally will save you money. Don't just look online, pick up the phone and speak to real people as often you can negotiate a better price.
Also there are several things to consider.
Are you buying a full set (four or five) and if so will your local supplier give an additional discount?
If buying on-line how much will your local dealer charge to fit, balance, dispose of the old tyres and does this include removing / refitting them to your vehicle (or is it loose wheels only)?
If it costs in to buy them on-line and have them fitted locally, how will you get them there (a full set of tyres can take up a lot of room)?
Have you considered a package? Makes it very easy as you just remove the old, bolt on the new ands then sell the old ones.
I have done this twice now (265/75x16 Cooper ST Maxx on Sawtooth and 265x75x16 BFG AT KO on Sawtooth), very easy and saves all that charging around.