Mahoney's is great. I've been to both their Winchester and Brighton locations. Brighton is probably closer, but the Winchester one is huge. I was there a few weeks ago. Tons of plants in every price range.

You could also check out Pemberton Farms on Mass Ave. They have a small greenhouse for indoor plants that's open all year.

Or if you want to try to find some free ones, there's a Facebook group called Somerville Plant Exchange that only allows free exchanges, no sales. Not sure if there's a similar group for Cambridge.