Is this an extract recipe? My understanding is that maltodextrin is mostly used for extract brewers to add body. All-grain and partial mash recipes would normally use different grains/adjuncts to achieve the desired effects instead.
Maltodextrin is an unfermentable complex carbohydrate that adds body/weight/mouthfeel with little sweetness (some say it's up to 12% fermentable, there is no standard for it so it may vary a little). Bodybuilders make shakes with it for post work out carbs since it doesn't break down as fast as dextrose in the digestive system (no quick spikes and drops in insulin/blood sugar) and doesn't taste too sweet. Anyway, getting off topic, haha
If the purpose of adding maltodextrin is to add body, Lactose (milk sugar) could be a substitute but my understanding it will give a much sweeter/smoother texture than maltodextrin.
Dextrin (Carafoam/carapils) would probably be the best substitute if doing an all-grain or partial mash recipe, it will give you extra body and head retention plus other grain benefits but won't be quite as "smooth". It will make the beer cloudier because of the protein but that shouldn't matter in a porter. Unmalted barley or oatmeal can add body too if you are not set on following the exact recipe or need to follow any style guidelines.
If you want to follow the exact recipe, I'd say Scoop and Save might be your best bet to find it around here or perhaps GNC?.