The one item that stands out as 'possible' is an Amulet of the Planes. You need to get a decent int bonus to use it reliably but it lets you and 5 companions travel the planes.
That doesn't mean you'd be safe once you arrive though. Knowledge skill : Planes would give you details on what you should expect and even targets for 'safe' zones you'll want to visit. Like on the Plane of Fire you might aim for the City of Brass because it will be slightly more hospitable than an endless sea of flames.
As a druid, you'll be able to use wild shape to help cope with most environments. The elemental forms are the most helpful for this. Though depending on the plane you may wish to use other forms.
One of the few planar type spells druids have is Planar Refuge which can be used to set up a base. Please note the 1 hour casting time, you need some way to survive that 1 hour.
Also druids get access to Fay Gate (8th level spell). I'm not sure if that would be good enough for you or not. I suspect not?
There is also the possability of using the Entice Fay line of spells to gain the cooperation of a fay creature that can travel the planes, but I couldn't find one under 18HD that could do it. Then again, I'm not an expert on fae creatures.
Transport via Plants technically never mentions that the target must be on the same plane if you know where a specific plant is. The ruling hinges on one's interpretation of the phrase "any distance", and may thus under certain interpretations depend on open planar portals existing *somewhere* in the plane you find yourself on that eventually lead to the plane you are going to.