Can I get new Invisalign retainers and how much do they cost?

I finished my express 10 Invisalign treatment. My dentist then asked for $500 to pay for retainers (the clear ones from Invisalign). He said I would be getting another retainer in 3 months & he would call me when they came in. I wore my first retainers for 3 months & then lost them. I called to see if my second set had come in yet, & my dentist said he made a mistake & I'm not getting any more retainers. To replace the ones I lost he wants another $500. Is this accurate? Do I have other options?