Sausages are traditionally made of meat with casings of hog, sheep, beef or collagen products. Vegetarian sausages and hot dogs are available in many stores, but you can make your own and use vegetarian casings, too.
Commercial vegetarian sausages are usually soy-based, but you can mix yours with any vegetable products that stick together well. Although they exist, sausage casings that are made of vegetarian products are not widely available on the consumer market, so an alternative is to make your sausages without casings. Making sausage patties is the easiest.
The company Meat Processing Products offers vegetarian sausage casings along with its meat-based products. These vegetarian casings are about hot dog size, they qualify as vegetarian and vegan and are listed as kosher- and halal-friendly. They have a taste similar to that of collagen casings.
Another company making vegetarian sausage casings is vegicase.com. They sell their products in large quantities, meant for the commercial market. If you are making your sausages in large quantities to sell, this is a good source.