How do you get monster fluid in Monster Hunter Freedom 2

Two ways.

1.) If you have access to wireless internet you can download an upgrade for the merchant that stands outside of the guild hall. Then she will often, but not always, stock them. If she isn't stocking them just do a mission and come back. However, they cost 5,000 to buy. I don't recommend this way.

2.) Kill a vespoid, hornetaur, or great thunderbug with poison. It can be either a poison bomb, or a poison weapon, but the poison needs to kill them. If they die from poison instead of exploding they will be lootable. Vespoids are easiest for this as they die very rapidly from poison.

The drop rates are:
Vespoid - 17% / 15% for G-rank
Hornetaur - 20% / 0% for G-rank
Great Thunderbug - 20% / 10% for G-rank

Those same creatures will drop monster broth as well, but only in G-rank missions.

I got these percentages from the boss/miniboss carve faq at http://www.gamefaqs.com/portable/psp/game/934395.html There is a lot of good information on that faq and others listed on that page. Hopefully that helps you out.
u can get it from killing hornetuars in some low raked quests do the fifty hornet one and ur sure to get at least 2 works everytime for me (im hr 6)