A good one for a Syrian is this one - it's a really good size and doesn't slope in too much. It's not that cheap but supposed to be easy to cut. Only thing is you can get an Alaska cage for less money!
It's 81 cm x 62cm
Go Shopping - Really Useful Boxes - 145 litre Really Useful Box
Alaska cage is 84cm by 48.5 cm so the RUB box has better floor area but the Alaska is cheaper
Hamster Cages | great selection at zooplus
If it was for a dwarf hamster there's the 78cm Ikea Samla box which is cheap but I don't think it's big enough for a syrian as the floor area is quite small - it slopes in a lot at the bottom.