At one time, 'Radio Shack' (known in the UK as 'Tandy') used to sell a 'Realistic' (own-brand) PZM. PZM (Pressure Zone Microphone, another description for boundary mic or 'plate mic') is a brand name belonging to Crown, and the Tandy version was an unbalanced, battery only PZM, manufactured by Crown.

If you can find one, it's a fairly straightforward matter to convert it to balanced line with phantom power... plenty of drawings available if you do a search.

Of course, you may find that it's perfectly adequate without the mod, particularly if you use short mic lines and haven't got phantom on your mixer.

AudioTechnica do a range of boundary mics from around 25 to 250 squids... avoid any branded 'Eagle', though!

If you happen to have a tieclip microphone (perhaps the type of thing people use with MiniDisk recorders), try clipping it to a piece of card lying on its side, as it will work fairly well as a boundary mic. You can hear a dramatic difference in the 'focus' of the sound compared with the same microphone in free air, and you'll also find you can get significantly higher level before feedback.