Start small. Find a flaw or equipment need in your own town's department and try to spearhead a rally in the community to raise money to correct it. Does your town's K9 officer have ballistic body armor for the dog? If not, try to raise some cash to buy him one. Or maybe a bicycle or two for some community policing.
Trying to raise enough money for a large department would be quite difficult to make a difference. But for a smaller department, some money for new vests, some advanced training for the officers, etc etc etc could go a LONG way!
Also, once you spearhead a couple of fundraising ventures and they prove effective and worth while, people will be more likely to donate. If some dude off the street hit me up for cash for the Police Department, I'd find it hokey. But if you organize it right and do a couple of very effective donations, you'll gain greater reputability for your cause...
Good luck!
American Veterans of the War in Iraq & Afghanistan
http://www.avwia.com