Where do I find the info I need to get the right spacers for my alloys?

I have a set of 17inch alloys I need to fit to a Citroen Saxo, but where can I find out what size spacers I need? I've seen loads of aftermarket spacers for sale online but no where can give me advice on how I find out what size I need! I'd just like to know where/who I should speak to to get what I need.