Dual conversion refers to the receiver and the manner in which it processes the signal. The carrier frequency is 75 MHz. In a single conversion radio, the electronics convert this frequency to 455 KHz. In dual conversion equipment, it is first converted to around 10MHz then again down to the 455 KHz.
The advantage is it allows for greater selectivity in the signal. The disadvantage is it adds cost to the system. Better performance generally comes with a price penalty.
The crystals are different on the receiver since dual conversion requires a different output frequency.
Regarding pricing, I have had good luck with a company in Madison, WI called Hobby Horse. They are a stocking HiTec dealer, offer great pricing and quick delivery. Their link is
http://www.hobbyhorse.com/
Another thing to remember is crystals are generally specific to the brand of radio - ie Futaba crystals won't work in a HiTec radio. Also your radio needs to be built/tuned to 75MHz. You can't take a 72MHz radio and convert it by just changing the crystals.
Paul