I have read that it is possible to install ADFS on a Domain Controller (although some do not recommend it). However, I cannot find much on installing on SBS 2008. Is it possible to install Active Directory Federated Services on a SBS 2008 Standard server?
According to Microsoft's compatibility Matrix for ADFS 2.0 it is officially supported on SBS 2008 Standard (see link below), if you can share your end goal we can provide more specific direction (sharepoint/office365/third party apps/etc)
Having said that, I would personally not want to expose a server that is out of support to the outside world to handle authentication (MS support matrix here https://support.microsoft.com/en-us/lifecycle/search?alpha=windows%20server%202008), but that's a separate discussion.