There is the obvious option of working with a realtor to find a space. Here are some non-traditional options:
Rent an apartment. Yes, a residential apartment. You'll need to find the right building to do this without hassle. Larger buildings with many units are usually cool with you running a business, for good reason. You pay on time, you are mostly quiet, and nobody is there overnight. Kendall square is probably out because of price, but there have been startups operating out of apartments in the Watermark building.
Shack up with a bigger startup. Leases are for 3-5 years, so most growing startups lease more space than they need. To manage costs, they sublease space, or even individual desks. One startup I know of with office space near Park Street is renting desks for $200/month (all spoken for, sorry). By joining them you get desks, chairs, and internet taken care of for you. Just be prepared to move out when they grow into the space. To find startups with extra space, go to the VCs: they always know who is renting office space, and how much money they are (painfully) spending on it.