If you what to buy a GPS receiver, a good place to start is in the sailing world, for instance by visiting a local store that sells yachting equipment. Many outdoor stores also sell GPS receivers and will offer advice. The scouts in your area will probably be able to help you.
The GPS market is currently in a great state of flux and it is strongly recommended to talk to somebody who has experience before buying a device.
Just some years ago, GPS receivers were still big and expensive. This is no longer so and there are now hand-held receivers on the market which are no bigger than a mobile phone and cost even less.
ESO staff and others who work in the Atacama desert at the site of the Very Large Telescope make sure to carry a GPS receiver when they travel in these harsh surroundings!
Current prices for hand-held devices range from about 200 EURO in the low end, to about 5 times this sum in the high end. Many come with a large palette of extras, including fixations and bags. More expensive devices are also available for fixed installation in boats, etc.
Watch out for the functions included (type and size of display), user friendliness (how difficult is it to use?) and robustness. How long will the batteries last?