See Boston Consulting Group, Deloitte, Ernst & Young, KPMG and PwC. And then the second tier below that.
They also offer the lead-in service of making your idea into pretty slides with their logo, so it's clear later that they endorsed the idea from the get-go.
I don't mean the tax/audit arm of those companies. They all do the generic management consulting. I left several off. Here's a market share chart: https://imgur.com/a/O1Urjkm
They also offer the lead-in service of making your idea into pretty slides with their logo, so it's clear later that they endorsed the idea from the get-go.