-direct dmg buffs (or mana to maintain dmg):
Mongoose (alch)
Winter Squid(fishing only in winter from september till march)
Mana Potions(alch)
Demonic Runes(satyrs felwood or azs)
(you can also get Mana Oil but the 2 above should be enough for almost every encounter)
+25 Agi Buff from Blasted Lands(do a repeatable quest in bl)
for your pet:
Strength Juju(repeatable quest in ws)
Attackpower Juju(^)
Scroll of Agility(kill mobs? buy from ah)
Ony/Nef Head World Buff(sw)
ZG Heart World Buff(zg quest hub island)
Songflower World Buff(repeatable quest in felwood, cleanse plant take buff multiple locations)
Silithus World Buff(only 30 mins so up to you if its even woth to get might not be because of travel time)
These are only, like mentioned, Buffs that boost your dmg i did not list the buffs that give +stamina etc
For Nefarian: Stratholm Holy Water for the undead add phase
And ofcourse Thorium Headed Arrows which an Engi can craft for you (if you are not already engi you should consider it anyway)
If you happen to be an engi Grenades are usefull and so is the Battle Chicken it wont boost your own dmg(only your pet) but the meeles in your group will be gratefull for it)