Paul recommends the 10″ (25cm) with 15-17 PPI. There are many different names on the saw plate but only one chief maker. They usually come in different lengths, but the 10″ is the ideal length. Also, small teeth (anything over 17PPI) is just too small and the cut is nowhere near as effective. It also becomes difficult to sharpen, requiring extra slim saw files.
Paul recommends new where possible for this saw as a secondhand one will probably need restoration, reshaping of teeth and sharpening. As these are smaller toothed saws, they are more problematic to sharpen which might not be ideal for a beginner.
There are several options online, one of these is this from a reliable eBay seller.