I have used DonorsChoose in the past. I’m not sure if their policy has changed there, but they have special policies for projects outside of conventional classroom materials.

I have used Donors Choose in the past, but we also supplement with fundraisers in our own building. We’ve had garage sales with donated items and all proceeds fund our field trip fund. Also, many of our local parks and libraries will pay the cost of buses through a grant of their own and then do not charge us a fee to attend the trip.

Another option is digital field trips. The Center for Interactive Learning and Collaboration (cilc.org) has a bunch that are free to places all over the US.