Mix your own.

Quickcrete portland cement and fine sand... add coarser aggregate as necessary. Plasticizer also really helps with finely detailed models.

I second the Quickcrete suggestion, I also have had very good luck using quickcrete cement mix, it gives you a very nice finish. 

Be sure to vibrate it.  Nothing crazy, you can just put a pad sander on the surface with your filled form to help the mix get into the corners...

Don't use too much water, as it leaves holes in the cured concrete. Just enough to thoroughly wet the mix. A little plasticizer helps but don't use too much of it, either.

I don't know about that Wood Guy. More water is better than not enough water.  

Best way if you are mixing small amounts is to get a digital scale and weigh everything precisely. Start with ratios from engineeringtoolbox.com or similar and adjust per experience.

Archanonymous, in my past life as a contractor I mixed and placed a lot of concrete and mortar of various sorts, everything from concrete counters to fireplaces and tiled showers, and I have a degree in structural engineering which involved a lot of concrete research, but I have not made architectural models out of concrete so I can't claim to be an expert in that niche.

I agree that not enough water is a worse problem than too much water, which is why I specified "enough to thoroughly wet the mix." Almost everyone uses too much water which weakens the mix (probably not a big deal for models) and leaves bubbles. You can fill the bubbles with a slurry mix if necessary.

a concrete vibrator should be good for tons of fun in studio, just make sure your prof and classmates can handle dirty jokes

Alternatively, you could get another type of vibrator and get the job done (for smaller models).

maybe save a few bucks and ask around?