The cheapest would be 3d printing, injection moulded is usually cost efficient for much higher orders think 1000min however I have seen ads for UK company that do small jobs for 10 or so I’ll recommend shapeway if you require SLA/SLS quality
your STL would most likely need modification to be injection moulded compatible but the company can do this
for the filament you can use the one marked as free to use otherwise there is page in here somewhere with filament supplier I use 3dfilaprint most of the time