Where can I find different stat potions in Realm if the Mad God? And of which can i find early on?

Stat potions are consumable items that permanently increase a specific stat (like attack, defence or speed). They are not soul bound so they can be traded between players without any problem.

There are 8 different stat potions.
Most dungeon and event bosses are guaranteed to drop a stat potion, the drop locations I have noted below are the most common ways to get them. Most gods in the god lands have a low chance to drop stat potions, so another easy way to get pots is to farm in the god lands, I shall also note the gods in the god lands that drop stat potions.

These potions are:

Potion of Attack (+1 ATT) which drops from flying brains, white demons and in the manor of the immortals.

Potion of Defence (+1 DEF) which drops from Beholders, Leviathans, Ent gods, Slime gods, in the Abyss of demons and in the Mad Lab

Potion of Speed (+1 SPD) which drops from Medusa, Djins, Ghost gods, in the Snake Pit and in the Cemetery

Potion of Dexterity (+1 DEX) which drops from Sprite worlds and The Nest.

Potion of Vitality (+1 VIT) which drops from the Abyss of Demons, the Hermit God and in the Cemetery

Potion of Wisdom (+1 WIS) which drops from the Mad Lab, Undead Lair and in the Cemetery

The above potions can all be dropped by Oryx (1&2) and event bosses. The following stat pots can not be dropped by these bosses.

Potion of Life (+5 Max HP) which drops from the Tomb of the Ancients and the Shatters.

Potion of Mana (+5 Max MP) which drops from the Ocean Trench, the Crawling depths, the Shatters and the Ice Cave.

These are all the stat pots that are currently in the game, running dungeons or farming in god lands is the easiest way to get them, but trading with other players for items works as well.

For your first dungeons which can give you stat potions, I would advise the Sprite World and the Snake Pit, later on trying the Cemetery, Mad Lab and Abyss of Demons are going to be fun and rewarding. I would stay away from the Ocean Trench, Tomb, Shatters, Crawling Depths and Ice Cave for the time being.

Source: My own experience from playing the game for a couple of years and the Realm of the Mad God wiki page