I'm very new in the land of hair extensions and I have just started exploring into it. This is what I do know. You can try looking around your local area for wig shops/hair supply stores. Usually they have them and prices range all over the place, depending on style and length. The longer it is the more it usually costs.
For a really cheap route, you can try synthetic, but it is not very durable, tangles quite easily, and can be slightly more noticeable in terms of hair quality. Upside is, humidity doesn't affect it, so whatever specific style you ordered in, it will stay in. It's a quick and easy fix.
You can check on ebay, but make sure to check reviews and snoop around a bit from the store you're purchasing from before making that initial payment.
I have heard great things about headkandy. They give you lots of hair, it is actually full head and will even help you out with sorting out the right color you need. If you end up with a color that isn't as close to your hair you can send it back and they will give you the right one. There quality is great, offering lots of coverage, very thick. Personally, I have not ordered from them....yet.
http://www.headkandy.com/
Here are some other links. You should check out reviews before purchasing.
http://www.dolliehairextensions.com/catalog/Clip_In_Hair_Extensions_Standard_Set-1-1.html
http://www.rapunzelrapunzel.co.uk/?ref=12
http://www.cliphair.co.uk/Full-Head-Clip-In-Hair-Extensions/
I hope that helps some.