It's hard to find a short-term vacancy in Boston for less than $250 / week. Most of the real estates offer apartments for rent for at least 6 months. Finally, one of the real estate's guy of Brookline told me yesterday that I should search CraigsList [http://boston.craigslist.org/sub/]. I do not know what "Craigs" exactly means, but the website was really helpful, however, most of the offers are apartment sharing that I actually do not tolerate!
In other hands, renting an apartment for 6 months or 1 year is a good deal in Boston. You can get a 1 bedroom apartment in Newbury St [Google Map] for less than 550$/month.
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Still looking for a separated apartment for short-term for less than 1000$/month in Boston. What a hard mission!