There are several websites, to be honest I think the market for foreigners in Milan is so small that I doubt there could be something like a "rental ads website with bias toward expats" 
Usually there are always two option:
1. Direct contact with the landlord
2. Through a real estate agency
Of course like always there are pros and cons: managing business directly with a landlord is usually cheaper, while a good real estate agency can ensure that the landlord is not trying to rip you off just because you're an expat, for example.
A few websites:
* http://www.immobiliare.it/ (you have to specify that you are looking for rents in Milan)
* http://www.kijiji.it/case/affitto/annunci-milano/affitto/?entryPoint=sb (direct entry point for rentals in Milan)
* http://www.casa.it/affitti (entry point for rentals, then you have to specify that is Milan)
* https://www.idealista.it/affitto-case/milano-provincia/mappa (direct entry point for rentals in Milan)
A word of advice: be sure to speak a very good Italian if you want to deal personally with landlords or real estate agencies, you'd be surprised of how many of them don't speak English or speak it very badly.
