How can I get a replacement crankset gear for Dahon P8? It is a 52 T with 5 screws.
I searched shops here in Belgium, but they have only an expensive whole assembly.
Any chainring that has the same bolt circle diameter (BCD) and the same number of bolts as your crank is compatible with your crank. The ever helpful Sheldon Brown has a page describing how to measure your BCD. The way you do it is dependent on how many bolts you have so I won't go into the details here.
While a chainring may be compatible with your crank, there are other problems you can run into...
Depending on your chain, you may or may not be able to use a chainring that was designed for a different chain, e.g., a 9-speed chainring with an 8-speed chain. Many people do this with success while many others have problems. It's probably easiest to just get one that you're sure matches.
You also may have problems going bigger or smaller than your current chainring. Whether or not this is the case depends on your front derailleur and the amount of clearance you have on your frame. Derailleurs have a minimum and maximum number of teeth they can accommodate. Check the specs on your derailleur to find out what these are. The frame clearance is a bit more guesswork. It's kind of hard for me to tell from the picture how much of an issue it would be, but if you're going to go bigger you just want to be sure that nothing's rubbing when the chain is on the ring. If you're dead set on a bigger ring, you might be able to get a little wiggle room through spacers or a longer bottom bracket spindle but it's usually not enough to be worth the hassle.
As Benzo points out in the comments, the chain guides also limit the size of chainring that you can use. Consult with your manual and/or an experienced technician to determine what sizes yours will accommodate. Or plan on replacing those as well.