If you don’t have decorative grasses, that’s ok. Just go to a park, find a woods or somewhere else, you can almost always find something that will work.
Another idea is to dry flowers from your garden. Here’s a great article on how to dry hydrangeas. http://livedan330.com/2013/08/26/tips-on-drying-hydrangeas/
Just look how this regular grass went to seed in a park by my house.
Be frugal, get some fresh air, take your kids for a walk. There are lot’s of goodies to find in the outdoors.