Here is a list of some websites that I find very helpful in my quest to read and obtain every comic book ever printed.

I use all these regulary and I will vouch for them personally.

First,  Mile High Comics.
I use them to get comics that I missed and back issues. Very reliable, fast and courtious.

Next I use My Comic Shop.   When Mile high doesn't have it,  these guys will. AND they have the lowest prices I have yet found.   They don't seem to mark up comics a month after they come out like most places.

Next is my new favorite,  Discount Comic Book Service.  They sell current comics, and trade paperbacks for almost 50% off!  You have to order in advance, but if you are a regular, that is no big deal.   I have gotten two months worth of comics from these guys and the service is perfect.  No problems.   With the high price of comics, these guys, selling at 50% to 40% off are a life saver.  Thanks Mike for telling me about this place.

I've bought a few things from Mile High before; haven't had the opportunity to check out the others.