Non-members cannot make the potion via Herblore, but can still obtain it by trading with other players or talking to the Apothecary. Players can take a limpwurt root and red spiders' eggs to the Apothecary in Varrock, who will make a 4-dose strength potion for 5 coins. No Herblore level is required for this. At current prices, purchasing this potion from the Apothecary will result in a -3,062 compared to simply buying the potion at the GE and selling the ingredients there as well.
One-dose strength potions can sometimes be found inside barrels.
It should be noted that using a dose of a strength potion will not stack with the passive boost of the Armadyl godsword.