It's quite hard to find good castanets online.  Even students shouldn't be tempted by castanets described as "semi-professional", because they're usually poor quality.  If you can't afford "professional" castanets, it's better to look for second-hand than compromise.

Castanets on eBay

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LP LP432 Professional Castanets 2 pair    US $26.95

LP Latin Percussion Professional Castanets Rosewood Double Set    US $41.00