You can get them as well via any number of comic dealers, such as Metro comics, which is where I originally go then. The new Buffy Omnibus series will also collect all of them in, I believe, Volume 3, which will soon be out.
I'm still missing one, but I got the others from a comic fair and eBay. Even got Amber to sign them for me, wahey! I asked her about the other one and she said she hoped I'd find it to complete my little set. What a sweetie... Happy hunting!
Actually I just picked up volume 3 today and those stories aren't included because the book basically follows the storyline of season 3. There's plenty of Willow interest, but looks like you'll have to be patient for the Tara stuff! jem