This is frustrating; I'm sure I've typed out lists of resources more than once but now I can't find any of my posts! Mostly we've used past papers for maths competitions at various levels, e.g., Mathematics Association Primary Mathematics Challenge, UKMT Junior Mathematics Challenge, Scottish Primary (and Junior) Mathematics Challenge, etc.; there are others in the US e.g. AMC8/10/12, AIME, MOEMS, and of course apart from the odd money question these are totally international, but I haven't internalised which of those is at which level. Alcumus is great too.

We've had quite a few threads on similar issues; here are a few in case there are more useful suggestions there:
http://giftedissues.davidsongifted.org/BB/ubbthreads.php/topics/49569/1.html
http://giftedissues.davidsongifted.org/BB/ubbthreads.php/topics/45283/1.html
http://giftedissues.davidsongifted.org/BB/ubbthreads.php/topics/82679/4.html

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