There is a fairly new Walmart on Turkey Lake Road which is probably closest to I.Drive. (Turkey Lake is the road the opposite side to International Drive with the I-4 in between). I always drive when in Orlando so am not that familiar with the bus services. A taxi might be a good option for getting to this store. Another Walmart Superstore on Sand Lake (road intersects International Drive half way down) is on the way to MCO airport. I know the Lynx bus goes up that road and onto the Florida Mall so it might be a possible option.
There is an old Target by the Florida Mall but other newer bigger ones also around slightly further away. I tend to frequent the ones at Hunters Creek or by Mall of Millenia, but I don't think these will work for you without a car.
On both the Walmart and Target websites they have a store locator function. Just type in your hotel zip code ad the sites will give you a list of the nearest stores.
Happy shopping.