The serious ones are out there! I swear! I think I honed my trick for ignoring the Bubble Gum Books back when chick lit became popular. I got so sick of seeing pink cartoony covers on the front tables of bookstores that I started glossing over them altogether.
I got started on YA books through the Internet. I started thinking that the novels I was writing were more YA than adult fiction, and so I decided to check out the YA section for the first time since I was 14 or so. Back then (early '90s), everything was either horror, romance, or "issue books." But there were so few issue books that a fast reader could tear through them all in a summer. The genre started changing a lot in the late '90s / early '00s, and there started to be more room for comic novels, satire, sci-fi, and just plain old contemporary novels without some Big Teenage Problem lurking within them. Anyway, I started reading YA writers' and readers' blogs, and some of the same book titles and authors' names kept coming up as being really good. Everybody was raving about Laurie Halse Anderson, John Green, Markus Zusak, M.T. Anderson, Meg Rosoff, E. Lockhart, and others. So I started checking those out.
To this day, I don't think there's a YA book I've read that I've just picked up off the shelf. Everything's been an Internet recommendation. I go into the bookstore or library with a TBR list and grab what I can find. I ignore the fluff. There's a lot of fluff, sometimes.
My library has a really good YA section, with new books coming in all the time. My local bookstores can be hit or miss. Borders and B&N have been hawking the series books lately -- Gossip Girl and its many clones. Sometimes I hear about a great new YA novel, and it'll be nowhere to be found at the bookstore. If you have an independent children's bookstore nearby, they might be of help, though some may not carry some of the edgier YA titles.