Are you sure you can't open it? Worth a try, 'cos even if you b***er it, you've not lost anything. Don't know the actual switch (maybe post a pic?) bu it's at least possible that the problem is just dust in the working bits.
But to answer your actual question, have you tried Miles machine tools? They have quite a lot of deWalt spares, and give excellent exploded diagrams of the tools themselves. If you put the model number of the saw into Google, it will probably bring up other similar suppliers - it's just that Miles seem to have a pretty comprehensive range (Stress that I'm not connected with them in any way)