I've had an iBert for four years/two kids, and I like it a lot. My older son and I did some things with him in it that we had to hide from my wife/his mother, and it didn't break. (Which may be an argument against buying a used one.)http://www.amazon.com/iBert-Safe-T-Seat-Front-Child-38-Pounds/
I got this one as a present. It actually didn't fit my bike because I'm a midget, but it's a reasonably good price and felt sturdy, not too cheap. Of course, Craigslist is the ideal way to go!http://www.amazon.com/Schwinn-SW74625-1-Deluxe-Child-Carrier/dp/B0018CX1AA/ref=sr_1_1?s=cycling&srs=2600190011&ie=UTF8&qid=1391820455&sr=1-1&keywords=schwinn+bike+seat
We had trouble fitting seats on any of our bikes. I recommend a trailer. Our kids (who are big for their age) started riding in a trailer at about 10 months. They love it.
I've used both. They have different purposes at my house. A short, easy ride with the chair means easy "conversation" with the passenger that isn't possible with the trailer. But the trailer is great for nap-worthy rides.