In the past I've thought buy online cheap and get them fitted somewhere but a lot of the time this can be false ecomony. You will have to then add the cost of fitting......and...a lot of garages are getting wise to the fact that more and more people are buying online and cutting them out .. and because of this will actually charge you more to fit tyres you have supplied yourself.
Infact a mate of mine in the trade actually told me this ...and he owns his own tyre garage.
So what I'm saying is.....It's always worth a call to your local tyre place to see A) what sort of deals they are doing themselves to supply and fit and B) what they will charge you to fit and valve/balance a tyre you are supplying yourself...then you do the maths and decide which deal is best.