OllieMarmot

NASA can accept donations from the public, but they are not allowed to ask for them or advertise in any way. If you send a check to the correct place, they will absolutely use it. I believe they are only allowed to accept money from American citizens as well, because taking foreign donations would create some interesting conflict of interests.

EDIT: Thanks for the link /u/nikkorizz. It looks like the money can come from anyone, but it cannot have any "strings" attached.

You're not doing yourself any favors here.

Here is an article discussing other possible targets. The object that they are currently looking to go to after Pluto would require 35% of the remaining fuel, meaning there would likely be enough to visit another object after that.