The following are suggested sources of blood which you may turn to if you can't find any on your own.
The best place to gather blood would be at Liar's Retreat(lies south-west of Dragon Bridge). The cave has many falmer and the corpses of all races. A good spot to find a high elf is north-west of Helgen. Between Pinewatch and Anise's Cabin you'll find a hostile Novice Conjurer. Alternatively you can obtain the high elf blood by traveling to Solitude, then give the store Radiant Rainment a tiny visit. This store is run by two high elves, Taarie and Endarie. You should kill Endarie as she doesn't provide you with any quests. Taarie will still be there to run the store selling the same items Endarie did. You can also travel to the Thalmore Embassy after the Diplomatic Immunity quest for an easy blood harveset. After you've gathered the blood, return to Septimus.
Note: Northwatch Keep guards are high elves as well. If you've already done the quest there to save a prisoner, you can just fast travel there and get your blood off of dead bodies.