In my experience you have to have built a relationship with an SA. In some cases, this can be done rather quickly, while in others it can take a long time. I think it depends on the location of the boutique, as well as the personality of the SA/Manager of the store, and, of course, how well you "hit it off" with that person.
In one boutique where I often shop, I was offered a SO the first time I inquired (!). In another, though, I have had to purchase a veritable multitude of small items, which so far has totalled a few thousand dollars, and though my SA there keeps promising to help me when order time comes around again, it didn't happen this past February.
In some ways, I personally think it's at least partially luck of the draw! I guess they want to know that you're a serious buyer, and some SAs/Managers are more willing than others to take inquiries at face value.