Netmarketshare offers an estimate of browser share, based on data from around 11,000 partners.
However, no browser usage source covers "most of web usage", as you put it. All sites that report browser use simply publish their own (or partners') browser usage statistics, which are unlikely to cover anything more than a very small percentage of Web users.
As such, it's best to evaluate browser support on a site-by-site basis.
Use the browser statistics of the website that you're developing for to determine whether or not you should support a particular browser, not the browser stats of other websites.
If you're building a new site or you don't have access to user browser data, support as many browsers as you have the time and expertise to develop for, and start collecting visitor data to build a better picture of your audience as soon as you can.