Pumpkins are usually sold whole, but you can sometimes buy wedges of larger ones. If you buy a whole one it freezes well if you roast it. This can be used as pulp for soups. The pumpkins available at Halloween are not good eating pumpkins as they are grown for size and hollowness. Butternut squash is widely available and is an excellent replacement for pumpkin in most recipes.
Look for small, heavy courgettes. If a recipe uses chopped courgettes you can buy budget courgettes where their only fault is the fact they are uneven sizes or shapes. They are not hard to grow if you have a little space and time.