The fair market value (FMV) of your HSA on December 31, 2017 can be found on the 5498-SA in box 5. However, the 5498-SA is not required to be sent to you until May, so you might already have it and you might not.
The 12/31/2017 FMV may be on the appropriate statement from your HSA.
However, most HSA administrators have an online presence, so you can likely go to their website and look the FMV up 24x7.
Worst case, just call them.
[Edited 4/1/2020 5:27 pm CDT - typo]