An eligible project is approved by the MiAmbiente agency (previously known as Anam), under the Division of Forestry Dictory, where information on companies can be found. You can also check with ANARAP, the national association of reforestation. The main requirement for an eligible project is to have been granted the Forestry Certificate from MiAmbiente.
Unfortunately, there's no a list of projects. Therefore, what you can do is find a project and request a Copy of the Resolution issued by the National Environmental Authority of Panama, of the forest registry. As you would need a lawyer to apply for the visa, I'll recommend checking with them the forestry project, also, many lawyers can help you find a government-approved forestry project.
There are several government-approved forestry projects, however, not all of them are for sale. The safest way to deal with this is through the attorney that will take care of your residency application since they can always verify if the land being sold counts with the required forestry registration.