E-bay have loads of Handheld gps's ranging from garmin to Magellan and more. As it's an auction site prices start from £20 but you're likley to pay in the region of £60 to £80 for a reasonable one. I've just bought one and it cost £60. It's in good condition and works OK, but eats batteries like the're going out of fashion. But I have ordered a mount and power cable from Southern Marine Supplies, 2 Chalky Road, Broadmayne, Dorchester, Dorset
They have a web site which has all sorts of GPS accessories.