I've bought both genuine and third part ones on Ebay.
I, of course, prefer the genuine ones although many of them have been used hard. You'll find your fair share with bent plugs and/or housings. They will generally work fine even with these issues, but may be loose in the socket and the LEDs may be hit or miss.
I've generally had good luck with the "blocky" plastic ones that look like a generic PC laptop charger. My biggest complaint with them is that the cords are short-both the wall cord and the cord from the brick to the computer. The wall cord is the same as on most PC laptops, so you can use those to get extra distance. I keep one of these in my backpack to avoid wear and tear on my genuine ones.
There are some generic chargers that look like the genuine Apple ones, but without the Apple molded into the side. I've had one of these-it was in a lot of stuff that a member here sent me, and was brand new and unused. It was also DOA(I didn't complain, as it wasn't part of the original deal and the stuff was sent to me for the(substantial) cost of shipping). One experience isn't enough to draw a conclusion, however, so don't necessarily avoid these based on my one bad experience.
I've yet to see an aftermarket charger with the ring LED. This may or may not sway you toward buying a genuine one.
I've been able to pick up chargers-both aftermarket and genuine-for $10-20.
One last caution-if you are buying a charger for a Titanium Powerbook, be aware that some genuine 65W chargers will not work. Because the charging port is located right under the hinge, some 65W chargers have a "barrel" that is too large to fit. I've had some genuine 65W chargers that worked fine, and others that didn't. All the generic ones I've bought fit TiBooks fine. If you're buying for a TiBook, you're probably safest to stick to 45W chargers.