If you're in the UK you could try looking at All Ponds Solution, I got a nice external filter from them rather reasonable last month
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I believe Practical Fish-Keeping also has lists of recommended retailers and things (Link is not visible to guests. Please register to view.
It might help with finding cheap things and some shops even do deals, where if you find an item at a certain price online they'll match or beat it, World of Water for example is one shop that does that with certain things.