I have not seen a manual for the M-208 on the web in pdf.
You can purchase one from Tascasm, part number 5700099900. Ask for the Tascam M-200 Series Mixing Console Owners Manual.
The 200-series are pretty straightforward, but the manual can't hurt. It is well written in Tascam's signature "welcome to recording" style. Always a lot of nice extras in their manuals about recording in general.
Parts Department
TEAC America
7733 Telegraph Road
Montebello, CA 90640
(323) 727-4840
(323) 727-7617
You may also see copies sold on eBay, but I can’t be sure of their standing as to copyright, not to mention the quality. That could be hit or miss.