be sure to have done that quest first or you will not be able to start herblore
well speaking of where to get guam.. you cannot find it, instead you kill monsters that drop "UNID" better known as unidenified herb.. and the chances of getting guam is common seeing its the lowest lvl herb..but people but unids for 1k-1.5k ea so you can probably make money out of it.
if you want guam maybe take off your ring of wealth(if you have 1) and it stops the chances of higher herbs but i dont think that works because they are all unids, making them all same value