I have built one computer, so I don't know of many places. But I use Newegg, from Canada (I live there). They have some pretty good combo deals, and sell every part, although sometimes they won't sell a part, but only say one or two, and only in a certain brand. Now, I have a few reasons I like Linux.
A) Freedom (as in beer)- I couldn't afford Windows 7, and it's free, so why not?
B) Power- I installed Linux Mint on a 500GB hard drive, with 8GB swap space. And Linux Mint takes up about, oh, say, about 500MB (I think). For this size, it is very efficient, I could do anything I could ever want on here comfortably. For example, to find the MD5 checksum of a 4.4GB file, it tales about two minutes. On m Win7 laptop, it would take about ten min and the system would be unusable.
C) Compatibility- I use Linux Mint because it is so compatible with Windows. All I did after installing and installing the updates, was install Adobe Reader, and the WINE Windows program executor. Other than that, it is full featured and just like Windows. I had to install no drivers other than the official nVidia driver for special effects. It comes with Firefox, and OpenOffice.org, which I use on Windows anyways and is compatible with Office.
D) Beautiful- Linux Mint, out of the box, looks great. And I love the special effects, that was one of the first things I enabled. It comes with a lot of themes and backgrounds, and there are plenty more at a link provided by it. One of my favorite parts is the opportunity to have four (default) workspaces, so four desktops to work on. Because of these, I almost couldn't go to Windows or OS X.
E) Software manager- You can install programs from one central program, which contains about 35 thousand different programs, all separated into categories.
And some advice: for your use, photography, I'd highly recommend a solid state drive (if it's an option) because you will see much better performance. I'd also recommend an Intel Core i5 or i7, as you get a lot more performance. If you don't want Intel, or just can't afford it, I'd stick with an AMD Phenom II x6, as they're heap and hexa core. If you'd like some more advice, send me an e-mail at shark.basketball(at)google.com, and I'd be happy to help.