Look under "Deals, Freebies and Resources" for the comics subdirectory. Looks like Comixology is offering downloadable PDFs, but I have not tried this. Dark Horse and Image seem to offer some free comics, I think PDFs, too, and definitely sell them.
http://thrillbent.com/ started by Mark Waid (
Kingdom Come, among others) has downloadable free PDFs of comics; I liked the series "Insufferable". Also Loveandcapes.com is a series and I've been downloading the jpgs and converting them to pdfs. And there are many, many independent comics out there, it should be fun to search for them.
Me, I've scanned a zillion (it seems) of my comics and saved them as PDFs. If you have a scanner with a feeder, you can hit the "Quarter bins" (cheap, older comics) and buy a lot, dismantle them, scan them, convert to PDFs, back them up several places
and read at your leisure. This is assuming the JBC can read PDFs.
updated: at one time Thrillbent comics were free, but the site has become popular enough now to charge for subscriptions. However, it seems a fair charge. I didn't check, but back issues (PDF format) of
Insufferable, for example, might be free.