A while ago, I posted about a site called Comixology. As I've been using it a lot lately to purchase my comics, I thought I'd leave a review.
To start with, it's easy to sign up for an account on the site and you can use Pay Pal or your credit card to purchase the comics. If you're in the UK, I'd recommend using Paypal to purchase the comics. Also, do so by using the web and not the app. You'll still be able to download your comics with the app by restoring your purchases under settings.
As for the site itself, the comics are organised by series, which makes it easier to find what you want. For instance, Swamp Thing has several series but if you clicked on Swamp Thing (2011 -), which is part of the DC New 52 reboot, you'll find all of the issues and volumes there for the reboot. They're also constantly adding old issues of comics and there are a lot of series that offer the first issue for free. They often have sales too. Navigating the comics is rather nice too as you can zoom in and you can set it so you just view a panel at a time.
If there's a downside to the website/app, it's that your account purchases can become quite cluttered. There's currently no way to combine issues together, and there's no way to hide specific series from view when using the app. This means that if you have issues from a lot of series, then you're going to have a long list to browse unless you know which series you want to read, in which case you can just type the name in. Using the web to browse your purchases is better though, as you can archive items and unarchive them.
So yeah, with the exception of the downside I mentioned above, I do recommend the site. You might be able to get the comic volumes cheaper on Kindle, but I'm not sure Kindle is as good for reading comics and I personally don't like the idea of having my graphic novels/comics with my normal book purchases.