That’s exactly what my employer does, still you need to plan with a certain budget up front (e.g. a day / 8 hours at minimum). Typically customers who don’t want/cannot afford a full support contract plan with a couple of days per year, and only pay what’s been logged for troubleshooting, bug fixing, etc.
Freelancers may hop onto the offer too, still I would suggest that they also plan with a minimum up front. “Some hours here and there” sounds a bit chaotic and not really something you can build a living on (imho).
I don’t think that you’ll find someone in here though, as this is a community platform, where users help each other for the fun and pride. Better go with platforms where you’ll put your requirements and specific offers, typically some freelance (bid) platforms. Beware of those calling themselves “Icinga experts”, as no such term nor certification exists. It is easy to find out though with a couple of specific questions going deep into the DSL or cluster functionality for instance.