Good castanets are expensive, but buy a quality pair and they'll last you for years - and when you're finished with them, you'll find plenty of students willing to buy them second-hand.  Cheap castanets mean a cheap, nasty sound, so invest in a quality brand right from the start and you'll love playing them.

Castanets on eBay

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I like Castanuelas del Sur's range, especially their "teacher's edition" model, which I think is perfect for serious students, too.

Why?  Because unlike most castanets, they're extra light and ergonomically shaped - designed for teachers who spend hours in front of a class teaching.  But if you're serious about mastering the castanets, you'll spend hours using them too - so you'll appreciate the benefits.  They're easier to hold, lightweight so you won't tire as quickly, and the sound is excellent thanks to their double sound box.  Recommended!

If you're on a budget, my second choice would be the double sound box fibre castanets.  They're the same weight with the same sound but a more traditional shape - and 30 euros cheaper!

Note: NEVER buy castanets described as "semi-professional".   I don't know why this description is even used, because in my experience, semi-professional castanets are often thin, or small, or have a horrible sound.   If you can't afford "professional" castanets, it's better to look for second-hand than compromise.