Is there some kind of milk-free sour cream available, to use as substitute due to an allergy?
There are cultured soy and coconut milk products (generally sold with yogurt) which can be used as a sour cream substitute; it's frequently advisable to add a little extra vinegar or lemon juice as these products don't generally provide as much kick as real sour cream.
Sour Supreme is one brand of soy-based, non-dairy substitute.
You can create many vegan variations with silken through extra firm tofus; from ricotta cheese on down to sour cream. There are many recipes out there for making your own sour cream.
The problem with some brands, such as Sour Supreme (whose product is very authentic to taste), is that (in responding to the vegan tag) some of their products contain casein (an animal by-product). If you're just lactose intolerant then full steam ahead, but if you're vegan that is a choice you will have to make. Some vegans have no objection to casein or other similar trace additives, others do. Some do not eat honey because it can contain animal parts, others do eat it.
The Tofutti Sour Supreme does not contain casein, and is a great vegan substitute for sour cream. Avoid Yoso, for some reason it is sweet, it was disgusting on tacos, I also found that both their sour cream and yogurt taste powdery.
As someone mentioned above there are lots of recipes for making your own using silken tofu, lemon juice and I can't remember what else (I remember making one from the cookbook La Dolce Vegan that was pretty good).
Another amazing veg product is Vegenaise, it is a mayo replacement, but it tastes great in place of sour cream on potatoes and some other recipes, depending on what you like.
Vegan Gourmet Sour Cream Substitute is dairy-free, casein-free, does not contain any hydrogenated oils, and is very delicious.