There is no direct report that will show you only the participant sponsors. 

A way to report on this is to run a People: All Donors Report. 
1. Navigate to Communities > Special Events > Friends Asking Friends > Kintera Thon > Thon Name > Reports
2. Choose People : All Donors Report
3. No need to choose filters
4. For Output fields, Choose "Sponsored Participant's First Name," "Sponsored Participants Last Name."
5. Run Report.