The problem is that they are a very generic design, you can buy alot of alloys that look like that without the centre caps you wouldn't be able to tell (unless you're a real alloy boff i suppose)..?
My best guess would be TSWs? as they make alot of alloys in the same plastic 5 spoke design.. unsure if they do 13s though.. sorry if thats not much help.
You could search round on the net for 13" 5 Spoke 4 Stud PCD100 alloys, if you measure the width of them this would help your search as well as the more details you have the easier it will be to narrow down.
Also I would recommend measuring the width of where the centre cap is, you may also want to try taking your car down to a Performance Parts center near you, they may give you a better indication and will be able to narrow it down if you say you want to find the centre caps for them..
Sorry if this isn't much help..