Do you mean online or locally? Nearly every store will have some sort of 12 gauge slug available.
Are you looking for sabots for a rifled barrel or regular lead slugs for a smooth-bore?
Bulk or just a few?
Shipping ammo costs quite a bit due to weight. Knowing your location would help (even just your province). If I recommend a store in Ontario, and you're in BC, that doesn't help you much.