If your not used to eating grains/wheat and you suddenly eat a large amount of them, then your stomach will be disturbed. you should gradually introduced grains/whole wheats, so your stomach can tolerate it. Veggies are a nice source of fiber, carrots, celery, broccoli, etc. Not too expensive, and also some fruits, such as apples, bananas. If you don't want to with the food route, then their is always fiber pills and mixes. I"m sure walmart and kmart have their own version of those which are not expensive.
That's my take on it, maybe someone else can give you more ideas