Excellent article entitled "Who Paid for the Trump dossier" in the WSJ by Kimberley Strassel, one of the best journalists in the business. It's a recent article, so subscription only, but here's the synopsis:
Fusion GPS are a political intelligence firm based in Washington who are paid to conduct "opposition research". They were hired by the Democrats to dig up dirt on Mitt Romney in 2012. Based on testimony to congress this past week, it now appears Fusion GPS have also been hired by Russia in the past to oppose legislation unfavorable to Russia. Fusion are now under investigation on whether they were acting as an unregistered foreign agent. This is the firm that was hired to dig up dirt on Trump and came up with the discredited "golden showers dossier".
Fusion co-founder Glenn Simpson has been resisting testifying to the investigation, and according to Strassel has made it clear he will only testify is he is not asked who funded the Trump dossier project. Interesting, eh. Strassel's conclusions "What if, all this time, Washington and the media have the Russia collusion story backwards. What if it wasn't Trump's campaign playing footsie with Putin's regime, but the Democrats. The more we learn about Fusion, the more this seems likely".