Would a 26 inch rigid fork work on a 27.5 rim and frame? I'm looking to replace my suspension fork for a rigid one since i currently only ride my bike on the road (i got slicks and road gearing already) but i cant seem to find 27.5 rigid forks sold near my area.
The key measurement you need is the axle to crown length required. It's the distance from the center of the axle to the bottom of the frame head tube. The axle to crown length takes into account both wheel diameter and suspension compression distance.
Axle to crown length is easy enough to measure with a fork installed in the frame.
Don't forget to also look at the axle offset - how far in front of a straight line drawn through the headtube the axle sits.
It really depends on the rigid fork. I experienced during the transition of 26 to 27.5 inch that some 26 inch forks are able to work with a 27.5 wheel. To fully answer your question some more information would be necessary (choice of tyre (width and height of the tyre), hub with and axle diameter, tapered frame/headset -or not). You could look out for some road or gravel bike forks, for example the nukeproof digger is a gravel-bike with a lot of mtb standards and 27.5 wheels - take a look at this bike and check if you could buy this fork separately. A quick google research has shown me that surly is producing 27.5 rigid forks.