Most science departments teach an environmental unit each year.
Working with a science teacher arrange field trips to various environmental/conservation sites in your area. This is a great time to visit local nurseries, state parks and forests, fish hatcheries, etc. These groups and businesses have a great deal of information that you can turn in to math problems.
In my case almost all of the student I have in math class also have the same science class. The science teacher had different students photograph and video the trips so those students that were not able to go on the trips could view them later in the classroom.
Some of the math problems we have done in the past include these areas:
1. The sizes of state park
2. The sizes of state reservoirs
3. The costs of fishing and hunting licenses
4. The clean-up costs of fish kills in the state
5. The amount of seeds and plants need by a nursery to in sure they met demand next season
6. The cost of tree removals and replacements