https://stonelinkpm.com/rental-search/

Samson realty is an east-side focused broker.

There’s a few more I browse once in a while…

Lyon properties https://www.lyonpropertygroup.com/rental-search/

https://www.nexri.com/. Nexus (I think they nickel and dime with fees, not positive though)

You can also go to the websites of those mill buildings directly to look for rentals if that interests you.