But there are bulb socket adaptors and similar gadgets with light sensors to turn on outdoor lights, and some of the little solar powered LED garden lights have photo cells to turn them off and on. Many of these are 10 $'s or so at home centers.
Those aren't the same thing, and they won't work. Most daylight-detecting circuits use CdS LDRs, which are far to slow.

You can buy phototransistors at Frys and Radioshack. They will work almost anywhere a photodiode will.

I have several hundred photodiodes in a big bag in my mad scientists' lab. If you PM me I'll send you some through the mail.

I have 2 homemade shutter testers, an analog one that uses a phototransistor and a digital one that uses a photodiode and microcontroller. I use external sources of light for both. Any concerns about the unevenness of the shutter tripping can be taken care of by putting the measuring photodiode at approximately the film plane, which you should be doing anyway. Since the photodiode is approximately a point source, any concerns about the shape of the shadow cast by the shutter are taken care of. You are welcome to worry, I suppose, about the light pulse being not-exactly-digital due to lens effects from leaf shutters, but I'm sure not.