This is a quick and dirty True Type render of a small caps version of Source Serif Pro (Still a work in process)
Only the normal and bold weights are rendered; if you want more weights, download and install the free Fontforge, which will happily convert .ofl files with OpenType small caps in to True Type files. Extra points if you make a GitHub repo with the converted .ttf files.
The font SCTest-regular.otf is a test to make sure a document processor supports true small caps. If, after enabling small caps (e.g. using the font name "SCTest:smcp" in Libreoffice), "abcdefghijklmn" are a sans-serif bold font, this means the processor supports small caps; if those letters are serif small caps letters, this means the document processor is faking small caps. The sans letters come from Source Sans Pro, available under the exact same license as Source Serif Pro.
THIS IS UNOFFICIAL! I will not keep this up to date with the changes to Source Serif Pro. If you want that, get Fontforge and do it yourself.
See also:
(This used to have a version of Source Serif Pro Italic, but since that has been released, it now has the small caps version instead)