Once you have this recipe, you can craft the mutation serum at a chemistry station any time you wish
In order to get a Mutation Serum Recipe you will need to have done Enclave quests until the point where you have joined the Enclave. Once there head to the Enclave Science Wing MODUS Vendor. This is the same vendor that sells mutation serums for 5,000 caps. Everyone one in a while this vendor will also sell the recipe for one mutation serum for 20,000 caps. The price can be lowered by certain Perks like Bargain.
The price is still very steep, so it's not like everyone is going to afford to get many of these, but it could be a nice investment if you plan on selling many of them to other players.
It seems that the vendor only sell the recipes very rarely, so you will need to check back with the vendor every once in a while to see if it is for sale. You can expect to spend a long time to find one, and certainly a very long time if you want to get all the 19 different mutation serum recipes that are currently in the game.
Serum recipes as dropsSome players have reported that they have gotten mutation serum recipe as random drops from level 80 enemies. The drop rate seems extremely rare, so I would not advise just grinding level 80 mobs for this single purpose.
Several independent players have also reported getting a mutation serum recipe from the Scorchbeast Queen, which is considered the final boss in the game. Again, the drop rate seems to be very low.