Not a stupid question! There are tons of ways to get ebooks, various costs. There are sites like the Kindle Store, the Sony store etc. There are tons of free (legal) ebooks where copyright is no longer in effect, and plenty of authors release books for publicity for free. Google searching for free ebook sites in the US is helpful to find the places with legit legal free downloads. The kindle and sony store, etc etc have various prices. LIke regular book stores, you can get discounted books. I've seen them as low as .25 - $1.00 per book. Most new releases sell for a couple of dollars less than a regular books.
I happen to live in a country where it is not illegal to share digital books, so for that I'm fortunate. I can read anything I like for no cost by using a newsgroup to find copies of whatever I like.
Libraries will let you check them out and they expire all on their own (how neat is that??). A great way to continue to read free. I know that Kindle allows you to have a list of Kindle "friends" (I believe its up to 5 friends). Basically you link your accounts in the Kindle store/software. It allows all of you in your "friend web" to share your books legally. Your books also stay stored on their server so even if you delete from your reader and your computer, you can grab the file anytime you like forever as well. I think, but not sure, that some other readers allow the same type thing. If I was using the Kindle, I'd definitely be finding people for my list that are avid readers with the same taste in books as I have to maximize my library.
There are also some neat free ebook sites that email you updates when new titles come available that are legal for free download. I get emails all of the time from various sites. Its also allowed me to find new authors I might never have known about which I find fantastic.