you want one with a VESA mounting pattern (thats like all of them, its an industry standard, i defy you to find one that isnt)
and they cost about £40 for one that stays flat, or about £60 odd for one that tilts
you could get an articulated one as well, but only if the TV is 25kgs or under
as for where, i do know of a place that sells them quite cheaply, but its a competitor as it sells a few bits n bobs of PC stuff too