As a collector, books are your best friend. My first bayonet book was Bayonets From Janzen's Notebook. I have more than 50 bayonet books and still recommend Janzen's as a first book.
My advice is buy the book first. With book in hand, eBay is a great learning tool, even if you never buy there. There are thousands of bayonets you can look at and practice identification. You'll have fun doing it and the exercise will help you settle in on which bayonets you are most interested in.
Where the best pickings are, depends a lot on where you live. In some parts of the country, yard sales and flea markets are good. Not so where I'm at in Northern California. I make most of my good finds at antique malls, gun shows, or on eBay.
Your timing is pretty good. Prices have softened in the sour economy and lots of good stuff is turning up for sale. The key is to know what you're looking at, so you can identify the gold nuggets that sellers failed to recognize and other buyers have passed by.
Lastly, be forewarned. Bayonets can be addicting.