wilko sell larger plastic boxes like that £5.99 each, thats abit less then most other shops, you can always try ebay and gumtree for 2nd hand ones.
Wilko Fish Tank/Vivarium 10ltr | Aquariums | | Domestic Bird & Fish from Wilkinson Plus
they are kind of middle sized, and have all sorts of coloured lids, you only have the colours in store or whatever is random online.
normall prices vary, i know pah sell somethign similar in multiple sizes, but they are a clear blue and shaped, and not as nice i think.