Had this problem recently.
M12 x 1.25 are the right diameter and thread pitch, 19mm hex obviously.
What you want to know is thread length, and overall length.
However, there are quite a few factors..
Are these OEM wheels (Citroen/Peugeot, e.g. VTR/S wheels etc) or aftermarket?
If they are Citroen/Peugeot standard wheels, you need flat faced bolts (including flat faced locking ones of course).
Are you using spacers? You will need longer well
bolts. (not nuts ;D

).
If you are using aftermarket wheels, you need a 60' taper. Without spacers you need 25-26mm thread length.
These are the ones I am ordering;
http://cgi.ebay.co.uk/Citroen-Saxo-1...item1c12e35b1d
Sounds simple, but was a mission just to find out the right measurements. There is no calculator, or seemingly definite answer for wheel bolts, such a simple, and yet important thing, even OZ don't know, they just say "they come with the wheels".. Well, that's all well and good, but if I'm about to spend £1000 on 19" wheels (I wasn't, but you could), any information I want to know you should give me.
My wheels are 15" OZ Superturismo's WRC's btw, when the bolts arrive, I'll let you know how i got on. Sure it will be fine though.