Keep in mind that although they're pellets, they can still contain ingredients that are harmful. I still see a lot of corn (or maize) listed in the ingredient of local pellets.
There's a list on GL which you can compare to:
Guinea Lynx :: Topic - Cavy Pellet Analysis
It's practically impossible to find a pellet in the Netherlands which isn't alfalfa-based (luzerne). However, because alfalfa is rich in calcium, it can contribute to the formation of bladder stones.
If it's available near you, I recommend Science Selective from Supreme. They're alfalfa-based, but their pellets are specially formulated to prevent bladder stone formation.
I'm lucky enough to have a vet nearby that sells Oxbow Cavy Cusine, which he ships from the UK. It's expensive (30 euro for 4kg) but since one of my guinea pigs is prone to health problems I'd rather stick to Oxbow (timothy-based). If there's any guinea pig likely to develop bladder stones, it's her. The last time she was operated on my vet took a x-ray and saw some bladder sludge formation, so now I'm very cautious about adding anything to her diet which is too high in calcium.