As far as we know, linen cloth is the first type of cloth you’ll encounter in World of Warcraft. It drops from any level 5+ humaniod mob to level 15-18. Then you will get wool cloth. If you are aiming for just linen, avoid mobs below 15 levels.
While the reason why you may need linen cloth can vary, it is most likely that it is for tailoring or first-aid. Some linen is also used in engineering, but not as much. You probably already know what the cloth is used for so I won’t go on.
linen dress white is available to all humanoids from levels 5-15. You will collect a fair amount cloth as you level up. This cloth can be used immediately or sold on the auction house for money building. For example, grinding linen cloth is faster when you have outleveled the mobs. You might grind them to get both experience and cloth drops. I fully understand this.
You want to grind for experience and drops. Find an area with a lot of mobs and respawn quickly, then start grinding. When it comes to linen grinding, there are a few options. One of the most popular is Defias Trappers Westfall if playing Alliance. The Razormane mobs, located north east of Crossroads, are a great spot for Horde players.
There is always a debate about where the best place to grind in WoW. The above are only suggestions. I believe that most mobs can be considered equally good, provided you keep within the appropriate level range.
You might consider Ragefire Chasm and The Deadmines if you’re a higher-level charater who needs to grind linen. This could be for any reason. The mobs in The Deadmines are more powerful than those in RFC, so you might not have as much luck with linen there. Remember that Ragefire Chasm is only for humanoids. The worms, elementals, and worms will not drop cloth for your killing.