Hi Pawan,
You can get Syncfusion.SpellChecker.Base dll in below location. Please find it below.
C:\Program Files (x86)\Syncfusion\Essential Studio\{Product Version}\Assemblies\Based on your Assembly version\
e.g
C:\Program Files (x86)\Syncfusion\Essential Studio\15.4.0.17\Assemblies\4.5\Syncfusion.SpellChecker.Base.dll
Or otherwise, you can follow the below help document to install the spell checker package in your application.
Please let us know, if you have any concern on this.
Regards,
Ashokkumar B.