You buy the weapons from chocolina. The first ones are free and immediately available when you have access to her (i.e. right after new bodhum). As the game goes on, upgrades become available in stores.
Yeah I saw the guide here as well. But it gets way too technical for my tastes. Constantly checking the dumb faq to see which node is next. Sounds like a hell of a chore to me and it doesn't seem worth it, not that much difference in the end. Sure as hell sucks out all the fun.
Arguing on GameFaqs is like the para-olympics; even if you win, you're still an idiot.
You can use the information about which nodes grant which bonuses--found prior to the node-by-node progression--to work out your own system where you won't need to be checking anything as you go. The progression in the FAQ is but one of many, many possible progressions.