You shouldn't need an adapter as the ports are supposed to be backward compatible. Your USB 3 device will simply run at USB 2 speeds when using the older port. If however your device requires higher power draw than a USB 2 port can supply, an adapter won't help.
Someone if trying to rip you off, you don't need USB 3.0 to USB 2.0. They are all backwards compatible down to USB 1.1 . The issue is if the USB device needs more power than what a USB 2.0 port can provide, than it won't work. USB 3.0 can provide 500mv where as USB 2.0 only supplies 250mv max. Some little external hard drives that run off 1 USB 3.0 port needs 2x USB 2.0 ports to run otherwise not enough power (or external power adapter).