Hi Andrew,
Vehicles are usually in various “groups” based on vehicle type, price and goodness knows what else.
I’ve hired a number of cars over the years all around the world, and on those occasions when I have requested a particular model when making my reservation, I have been told the various vehicles within each group are allocated based on availability. A couple of times I have asked at check-in and got the vehicle I wanted but mostly I am told the one I want isn’t available.
Perhaps you could email a few rental car companies to ask if they have i30s. You could still be allocated any vehicle from that group though but knowing they have i30s is a start.
If you make a booking perhaps you could email the company a few days prior to pick up to request your preference and ask again at check-in if need be.