This is pretty defineably public record:
https://en.wikipedia.org/wiki/Donald_Trump%E2%80%93Russia_do...
notably: it's not the GOP the organization that initial funded the oppo, but a private news org with conservative leanings. and the FBI did not fund the dossier, but were provided it during its creation. (side note: who cares)
When it comes to corroboration: I would think the special investigation is good enough evidence that the claims of Russian cooperation are being taken seriously, no? Significant is a weasel word but there is actual smoke here.