Catherine,
Bargain-hunting ebooks is a bit more complicated than looking for low-cost paperbacks. While there are thousands, maybe millions, of self-published ebooks for free (and some of those are very good), just grabbing random free ebooks means wading through a lot of horrible crap. (And I say that as someone who doesn't mind wading through the digital slushpile; if you prefer to forego the dubious joys of learning twelve new ways to inflict commas into sentences, you'll want to stick to books that have at least been waved past an editor.)
If you mention what kinds of books you like, people can try to recommend sources for those genres.
The
Baen Free Library is an excellent place to start--Baen releases a number of its titles free; often these are the first in a series, to entice you to read more. They've also put out a number of CDs bundled with new hardcovers, which contain an entire series or collection of ebooks--and given permission for the CDs to go online.
The Fifth Imperium has the whole set. All of Baen's other ebooks are available at their store for $6 or less each.
If you like romance,
AllRomanceEbooks has a huge collection. I stick to the ones without DRM. ("Digital Rights Management," which can only be read on registered devices; you have to make sure your account settings are all set up correctly before you buy and it can be difficult to transfer those rights to a new computer or reading device.) They have a newsletter with regular freebies and low-cost promo titles.
For some other genres,
Backlist eBooks is a site put together by authors who reclaimed the rights to their published books and are self-publishing titles under their own terms. Most of them are low cost and available in several formats.
You can check out Mobileread's own
Deals & Freebies forum, in which people post bargains they've found, and usually mention which bookstore (and therefore filetype) in the title.
I read a lot of fanfiction, which is free, and the
Archive of Our Own has an easy "download-as-ebook" option for all its fic. If you're new to fanfic, pick a
fandom you like, sort by # of hits to see what's most popular, and choose something with a lot of kudos or comments or both. (There are excellent works with low hit counts as well, but figuring out which ones those are is a bit more hit-or-miss.)
Nonfiction's harder; less nonfic gets converted to ebook, and there are fewer independent publishers releasing those titles. (The larger publishers, as far as a lot of ebook fans can figure out, are really trying to kill the ebook industry; they keep prices high in order to reduce sales to "prove" that there's more demand for paper.) Googling for "free nonfiction ebooks" will get you links to
lists of suggestions, but there's no neat repository of recent low-cost nonfic.
I keep finding fascinating texts at
Archive.org, but those are definitely not recent. (Also, they're not in the best format for ereaders; most are scanned PDFs that don't resize well.)
I hope you find some ebook sources that work for you. I have an older Sony, and I read on it *constantly*--mostly free ebooks, and my personal cap for fiction is $6 per book. I get a lot from
Smashwords, but I don't mind wading through the digital slushpile.