The early Sigma gunsoften give unreliable exposures in TTL mode, so only get the very latest Sigma gun ifindeed itsa Sigma gun your going for.
The Latest model is called the Sigma EF-500 DG super SA-N.
You can also use manyMetz flashguns in manual mode, such as the 40mz-3i and the 54mz-3.
The best Metz SCA adapter to use with the SD9 or SD10 is not at all critical, if you set the guns manually.
However,it has recently come to light that CanonE-TTL may be the sameas Sigma S-TTL, posing the tantilisng possibilty of using Canon EF flashguns on the SD9 or SD10 in full TTL mode!...Thisstill need to be confirmed though and I'm still looking forsomeone totry it out.