I don't know of any general biz forums besides the Bank of America small business forum and it's pretty dead over there. I have spent time over the last week or so on sites for Lawn Care and Snow Removal professionals, Machine Shops, tractor hobbyists and the vendors that support them, field spray equipment operators and others.

My methodology is: pick a business domain and then google it to see if it sounds interesting and if there's an online presence devoted to it. There are hundreds, thousands of businesses in every city in the US and their trucks and vans are driving past you all the time with their names proudly emblazoned on the sides. No need to brainstorm anything!