I am Trevor and I am a Guru here at Shopify! :-)
In the UK, you can purchase the hardware from 1 of 2 links which I have included below which should help!
I have also included a great doc below which will list all the supported hardware which you can use with the Shopify POS.
I hope this helps!
Hi again Sowseeds,
Just came across your questioon. I can personally vouch for this supplier as the owner is a colleague that I've kown for a while and work closely with. If you haven't found your solution, ask for Charles Mitchell and you're welcome to tell him that I referred you.