As per title. Urgent.
L’Occitane probably has it. There are lots of stores and outlets around Taipei.
If you run a search for 薰衣草精油 on PChome, Ruten etc. there are online sellers like this and this. I would expect places specializing in essential oils and fragrances like L’Occitane or like the little shop across the alley from the Earthtree down by Yongkang St. to have it too. (Earthtree is at Xinsheng S. Rd. Sec. 2, Lane 30, 35-1; that’s an alley parallel to and one east from Yongkang St…)
Last time I looked for essential oils I was surprised to find relatively cheap deals in the department stores (Shinkong Mitsukoshi?) between City Hall MRT and Neo19 / Warner Village. I don’t remember exactly where, but looking through there I found several shops selling them… with the best deal being an Austrian 
manufacturer.