We recently purchased one from Amazon called:
EcoBasic Ultrasonic Retainer Cleaner, Retainer Cleaner Machine for Dentures, Mouth Guards, Braces, Toothbrush Heads, Jewelry with 200ML Capacity, 45kHz Retainer Cleaning, 4 Modes for All Dental Appliances
Happy with it so far! Currently has a coupon to bring the price down to $42.50.