You an dehydrate tomatoes - plum tomatoes don't have as much water content, so they are a good place to start. Then use a food processor or blender to make a powder. You can also buy it - the related companies The Spice House and Penzy's has it. Store in your freezer though, that stuff loves to grab all the moisture it can out of the air.
If you ever come across a brazilian market ask for a mandioquinha/batata baroa and try the same dish. Tradicionaly you would boil them instead of baking it... But you're the chef
