At my house we have a few 'store bought' playdough toys (birthday, x-mas gifts, etc.), but some of the best toys for playdough are plastic forks and knives. They are very inexpensive (we usually have them lying around the house in drawers, cabinets and such). It is also easy for everyone to have the same thing (with 4 boys it is hard to have the same thing without buying 4 of whatever). If it gets broken or lost (it is easily and cheaply replaced).
We also have inexpensive cookie cutters (purchased mostly at the dollar stores after season clearance sales). We can get a package of Easter (or Christmas or whatever) cookie cutters just a day or so after the holiday - greatly reduced. Even if the cookie cutters are different shapes we have enough for everybody to have one (of basically the same thing - they all have cookie cutters) and then they can learn to share the different shapes.
Also, a big thing at our house (we do it every so often) is large appliance boxes. We have a friend that works at an appliance store and if we ask for a box we can usually get one. Yes, that's all - a large cardboard box. They boys love it!!!!!!! They will play with it for days (until it is destroyed). They usually play with it as is, but you could let them color or finger paint the outside of the box, you could cut windows, doors, etc. You could turn it into a car, train, house, or whatever. I have found out that with boys it will last longer if you don't cut holes in it.
What do you substitute for 'store bought' toys?