So you have found boards, they're either not acceptable or available. Got it.
So it appears your specs don't align with the market choices. Time to think outside the box? Perhaps go half custom and address the difficult portion of your needs, I2C.
Find a suitable parallel input relay board. Then, build a small custom board with a MCP23017 to convert I2C to parallel such that it mounts on the relay board header. You could have these designed (if you're not able) and built at nominal cost if you're building any quantity.
If you want din-rail mounting, have a look at Phoenix Contact, they build a full range of mounting clips and pic board holders.