A potion is neither a weapon* nor a spell component** so drawing it during a grapple is not possible.
Well, sort of
*Pretty much anything can be used as an improvised weapon, but that's going to be a tough sell to your DM since a potion bottle is very small:
A typical potion or oil consists of 1 ounce of liquid held in a ceramic or glass vial fitted with a tight stopper. The stoppered container is usually no more than 1 inch wide and 2 inches high.
**Some spells, such as tenser's transformation, require a potion (in this case, of bull's strength) as a material component. The grapple rules do not require you to draw a spell component for the purposes of casting a spell:
You can produce a spell component from your pouch while grappling by using a full-round action.
So you could draw a potion of bull's strength if you had placed it in a pouch beforehand, because it is a material component, and then drink it instead of using it to cast the spell.