I'm finding this tricky. How much are you willing to spend? How many rats do you have?
I highly reccommend, if you can afford it, the Freddy Savic 2 (I purchase mine at Pets at Home) and it's approx Â£60-Â£80.
You can try ebay - but I've not really found many good ones on there.
Are you any good at DIY? You can always buy a good sized dog crate, cover it in hardwire mesh and create levels, hammocks etc inside. They're usually a lot cheaper than similar sized cages - but do take more effort to set up. Try, www.zooplus.co.uk
i notice you are in scotland (i am too) and i wanted to suggest having a look on freecycle and gumtree for your local area.
i found my Rats on gumtree needing rehomed because the couple couldn't look after them anymore and i got the cages with them for free, however whilst i was looking before i got my rats i noticed quite a few cages being offered on gumtree and freecycle for free or really cheap so you should check there if you are on a tight budget.
and if you haven't got any rats yet you could always do what i did and help some rats that need a new home and you might end up getting them complete with cage for free!
plenty of people will buy a baby animal but not a lot of people are as willing to take in an adult especially when they don't know the animals history, so if you are willing to adopt it opens up extra choices.
where abouts in scotland are you? i might also be able to suggest other things depending on where you are.