Florence has a lot of flea markets, so it's just a matter of going to the right flea market to find what you're looking for.
- Mercato delle Cascine - open on Tuesday 7am to 2pm only. This place pretty much has everything. Located on the northern bank of the river Arno.
- Piazza dei Ciompi - open daily. You should be able to find antique postcards, Best day to visit is the last Sunday of the month when the flea market expands to the entire piazza and adjoining streets.
- Santo Spirito - open daily for household stuff, fruits and vegetables, but hosts an antique market on the second Sunday of the month.
- Fort Basso Market is another one that may be worth checking out.
Good luck!