Haste potions are labeled as "very rare" in the Dungeon Masters Guide of DnD 5e. That's the same rarity as weapons with a +3 bonus and is intended to be a possible reward for players above level 10. Additionally most magic items aren't available for sale and even common magic items like healing potions might require some quest before the alchemist is willing to sell them to you.
And fun fact, if you want to craft a haste potion, you need raw materials worth 25k gold for a single potion.