We got ours from my Grandmom-in-law's basement (they belonged to her grandmother, and still have the zinc lids, so cool!).
But other favorite sources of jars:
* Cup sized mason jars that once held candles or pesto.
* Classico (and many other tomato sauce brands) come in good, sturdy canning jars large enough for smaller kefir runs or a small batch of ginger beer.
* Gallon sized pickle jars, which are pretty good for kombucha because the neck doesn't come in much.
* Swing top jars from IKEA or European import stores.
* Recycled swing top bottles from fancy sparkling lemonades, or my favorite root beer (Virgil's).
Most of these I just pull out of the apartment complex recycling bins, I go walking the afternoon before the recycling trucks come and clean them well. Only trouble is that sometimes they're orphanned from their lids.