This allows for absolute quiet radio. Keyboard lockouts - this allows inadvertent pressing of keys while in the flight. Dual band radios, as mentioned above, can operate like repeaters, i.e. receive on one band and transmit on another. Since a repeater transmits on one frequency and receives on another, your radio must be able to do the same. Even though this is standardized, there are some non-standard repeaters in the world.
Some radios permit non-standard offsets.
Mail order is probably the largest source of products. Another great source for buying equipment is Mike Kelley at Kentuciana Soaring. Mike is a hang gliding pilot who not only knows what you need, but he has a reputation for beating your best price. The Ham community knows where and when the next one is. They're great fun, and full of fantastic bargains.