I sourced several of these Sports Direct branded Giant Calculators - the same size as the green one Lance uses in his videos. You can buy them direct from Sports Direct for £4 but there is a £5 postage. I bought 10 from Sports Direct and they came in a bag with no protection and 3 were smashed.

This is sold at cost to me but is only really value for money to you if you are buying something else and adding this to the basket. Feel free to email me if you have an enquiry about them.