Hey, As for the yellow slimes there isnt really a great area i have found for farming them. There is the cave north of Hurnscald, but it is always very busy! It is also more of a spot for red slimes then yellows, but yellow slimes are there.
There is also the bandits cave (the area where you farm bandits hoods for the bandits quest), there are a little less yellow slimes there, and they are more spread out but this area is much less busy!
As for getting a warlord plate and warlord helmet, are you planning on being an archer, mage, or warrior?
If you are planning on being an archer then you will do just fine with these quest rewards.
Forest Armor (+10 Defense, +3 Dexterity)
1 Cat Ears (+1 Defense, +3 Agility)
http://wiki.themanaworld.org/index.php/Bandit_Quest
http://wiki.themanaworld.org/index.php/Cat_Ears_Quest
If you are planning on being a mage then warlord plate and warlord helmet are needed even less as magic can not be used with them
If you are planning on being a warrior i recommend you get
Candle Helmet (+16 Defense, -1 Agility, +1 Intelligence)
http://wiki.themanaworld.org/index.php/Orum_Quest
As it is almost as good as warlord helmet
Your choices for armor as a warrior are limited, you can either start with chain mail and suffer through collecting coal/iron for more then warlords plate or jump right to warlords plate.
Note that with Forest armor you can easily farm yellow and blue slimes.
But in order to build a warlords plate the total cost is
*75 iron
*180 coal
*115,000 GP
Its a long time to wait.
As for 'when' you should start saving coal/iron, that's easy! at level 1.
Try to get warlords plate at around level 50-60.
And if you are a warrior don't forget steal shield either!
Dont worry about warlords helm till last.