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Rockstar Print http://rockstarprint.co.uk/contact.php
I purchased them from Rockstar Print. I am sure that there are many online that you could print.
You might want to add a zero. It's a typo. 500mm.
Sure thing, yes both just go direct to the Pi: https://www.amazon.co.uk/gp/product/B00UV9G8OC/ref=oh_aui_detailpage_o09_s00?ie=UTF8&psc=1