They are relatively rare and also highly desirable at this time. Not enough exist to warrant having large amounts of spares stockpiled or reproduced.
There are no Isetta Junkyards. Actually there are, but they are now called "private collections" and they are not sitting in a lot where you can pick parts off them.
There are a number of people that hoard these little cars over time, filling barns and other buildings on their property. They are not interested selling any parts.
The best way to source parts for a reasonable price is to take time and try to attend meets and events in your area.
You will meet other like-minded people. If they know you have a car and are trying to find things, this is when you will find people willing to part with items for more reasonable prices.
