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Hi there, you can find the imaging unit here: https://www.tonergiant.co.uk/product/Lexmark-500Z-Return-Program-Imaging-Unit-50F0Z00-19206/ and the toners are any of the top 6 products on this page.
The difference between the toners is the top 3 are compatibles and the next 3 original Lexmark. They each print a different amount of pages so you can pick any of them and they will work correctly but some will print more than the others (the number of pages can be seen next to the black square on each product).