Navajas are the traditional Spanish folding knife. Like American slipjoints, there are a few different patterns:
-Albaceteña, probably the best known onw, with the curved handle and a blade similar to a clip point.
-Capaoras, similar to Albacete navajas but with a broader blade with lots of belly. "Capar", the verb from which the name of the knife comes, means to castrate.
-Machete, one of my favourite patterns. It has a spear point blade and a rectangular or slightly oval handle (looks like a very big and fancy SAK). They make awesome users and the smaller ones are beautifull gentleman's knives.
-Taconcillo, with a spear point blade and the traditional curved handle.
There are a lot more variations, these are the ones I own. I bought them on a trip to Spain in a place called Cuchillería Simón, which is one of the oldest knife stores in Madrid, they have several very good makers working for them in Albacete.
There are some excellent custom navaja makers, my favourite is Mr. Expósito. He does some excellent work. I have his card somewhere... I'll post his contact info when I find it.
I don't know if there are any good places online to buy navajas, I wouldn't buy them from one of the big Spanish production knife companies. Hand made ones aren't much more expensive and are a lot nicer.