"Host Families" typically only work with educational establishments or language academies directly so if you're in Madrid to study Spanish I would ask the institution at which you're studying to see if they have participating host families.
Apart from this, you could check http://www.VenAMiCasa.com
for bedrooms in (typically) "family homes". These wouldn't necessarily be "host families" but rather just bedrooms in a family's house where some food may (or may not) be offered but I don't think it's intended so much as a cultural exchange as it is, simply, that they're renting a room that would otherwise be empty. These rooms are usually rented by the night but no doubt they'd give you a monthly rate. Good luck!
Anyone else have suggestions? Saludos, MadridMan
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