Another comic viewer is Comical. It's pretty good. Just google it. It's free.
As for site's to get comics. DC is great at giving away old digital comics.
For free digital comics from DC here's the link.
http://www.aibq.com/catalog_1.php
They are free. Marvel is not so good about giving away old comics. You can buy collections from this site
http://www.digitalcomics.biz/group.asp?grp=14 . These are in PDF format and can be viewed with Adobe Reader. I have 40yrs of the X-Men. They sell it on DVD. There is every issue in sequence for the last 40yrs on it.