The problem can be solved with common fluorescent lights. These are  available or can be ordered at local hardware stores.  The best size is four foot long fixtures with two bulbs. Common seed planting trays fit well under this size fixture. Some stores offer “grow bulbs” that provide a correct spectrum of light but standard bulbs work fine.

We mount the fixtures on wire or cord hung immediately next to a large south facing window. Note: in the photo the wire has several loops tied in them that hook over lengths of bent coat hanger wire attached to the fixture. This allows us to easily change the height of the light. Move the turned off lights / fixture high during the day for full sunlight and lower them to just above the seedlings at night. The closer the light, the better, as long as the lights do not touch the plants. We usually drop the lights just after sundown and turn them off at around 9:00 PM. This extra duration of light tells the plant that it is early summer rather than early spring and the seedlings grow healthy and are ready to transfer when the time comes.

The lights can also be used in fall when the duration of sunlight dwindles and the plants begin slowing down fruit production. Extra light will keep production up a month longer or until frost decides the issue.