If you can find tthem on Ebay, the old metal scarabs are cool. I bought 20 for $20, and then used 2 per base to boost up my scarab farm to 25 models.
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I'm resisting the urge to purchase a couple models just to paint.
Man these models are freakin awesome
In addition to the suggestions already given, you could also buy PuppetsWar's Cyber Beetles ($6.60 for 10). They're not too detailed, but then again neither are GW's.
http://puppetswar.com/img/p/69-197-thickbox.jpg