Oh wow, I'm having so much fun with this mode in Wii Sports Resort at the moment.

I've got just over 60 of the markers so far.
One obscure white marker I can think of is the plane formation that you see over the ocean sometimes - best to try to find this at night when the planes are lit up, because it moves around in the sky instead of staying in one specific location, and so can be tricky to spot.
Otherwise, your best bet is Google, my friend.