Where to get benchmark data for niche roles?

Hi all

After some advice please - where do you look for benchmarking salary/benefit data on niche roles?

We have 3 individuals in what I would call niche roles - no one we're aware of, even in competitors in our industry, have roles with the same scope/responsibilities and in some cases nothing even remotely similar. 

We could use their previous role, benchmark that and then just 'uplift' as understandably they are pretty key to us, but this makes me uncomfortable in terms of evidencing these salaries/decisions made. 

Typical online sites never find their roles. Suggestions aren't close enough to be accurate. 

Does anyone do anything differently when you've got a niche role? 

Many thanks,