Mites in my leopard geckos vivarium?

If the mites are coming in with the crickets you should see them in the tubs that they come in. If they are in the cricket tubs change your supplier. Check your substrate too they might be coming in in that. Also if you use newspaper that can sometimes get paper mites. I get them occasionally with my lizards. Thoroughly scrub his tank with reptile disinfectant. Every nook and cranny and the lizard too! Give him a bath (probably not in disinfectant) and make sure he's not harbouring any in his armpits. Hopefully that should get rid of the little blighters for good!