The manual lists the Yamaha FC4 or FC5 as being compatible, but the FC4 has since been replaced by the FC4A. The FC4A is shaped like a piano pedal, whereas the FC5 is more of a square shape and is less expensive. Also, if I'm not mistaken the FC5 is designed to click on or off like a switch when you depress, so you can turn on sustain and leave it on without having to keep your foot down on the pedal, whereas the FC4A will sustain only while you are actively depressing the pedal.
If you like the FC4A style of pedal but want to save a little money, just about any sustain pedal made by other companies should also work. However, you'll want to avoid the ones that are "half-dampening" pedals, because the DGX-305 can't use that type.