For years I have bought my tyres online as they are nearly always cheaper than buying locally. There are some good deals on eBay. But as has been said, do check where you can get them fitted as some places won't do it.
Also, you can save yourself a packet by avoiding leading branded tyres like Michelin, Bridgestone or Pirelli etc. Many of the budget brands are very good these days and unless you drive like Lewis Hamilton or do very high mileage, you'll hardly notice the difference lol.