For the most part, if you need insurance to enter a country, they will sell it at the border crossing. There are a couple exceptions, but once you get on the road you'll figure it all out.
You'll probably need
liability insurance for the USA, generally bought in the state that you bought/registered your bike. Every state has different laws. So get it, you'll need it. However, they probably won't sell it to you, you'll have to get some sort of international policy that's good for the USA. About the motorcycle license, just tell the cops that its good for a motorcycle, they will not know the difference, well unless you run into a cop that is an Israeli, they he may know what he's looking for on the license.
liability insurance for Mexico (not required but if you are in an accident/vehicle contact event and you don't have it, they throw you in the clink until everything gets sorted out) www.bajabound.com
or you can buy at most borders, just before crossing into Mexico.
In Central America, if its required, they sell it to you right at the border crossing for 10 to 15 USD. El Salvador, Honduras, Nicaragua, Costa Rica, Panama, IIRC.
South America can be a little more tricky. Some countries require you to have insurance by law, but will not require you to show it before entering the country, nor have a convenient place to buy it.
Colombia requires an insurance called SOAT. The trick is that it can be hard to find a 3 month moto policy, and the only border that I crossed where I could buy it right there at the border was in Cucuta (VZ border) At other crossings into Colombia, I would enter the country, then try to the find the SOAT in the first larger town I came to.
Venezuela requires it for entry, but would not sell it to me at the particular border with Brazil. I had to get creative with that one.
Ecuador, Peru, and Bolivia I had none, but some folks insist that its required for Peru, although they do not require insurance for entry in these countries.
Argentina requires it by law, sometimes for entry, but some borders will not have it available to buy. One border in Argentina where they did ask for it, was when I was coming from Uruguay to Argentina. They could ask for it at other borders as well, it all depends on the day and the aduana official.
I hear its required in Chile, but anytime the Carabineros stopped me and checked my paperwork, they never once asked me for insurance.
I cant remember about Brazil, but I know that I did not buy any insurance while I was there.
French Guiana, Suriname, and Guyana all require insurance. For Suriname and Guyana, its easy to get and cheap. French Guiana, not so much. They want to see a Euro green card policy that is good for France. The problem is that the only one you can buy there is good for a year and costs over 800 Euros. I had to get creative with that one as well.
As far as a third party insurance that's good for all of the Americas, if it exists, its probably not worth the paper its printed on, since no one in Latin America will accept it. So avoid wasting money on some third party stuff that claims to be good for all the Americas. While its probably accepted in the USA, its probably not worth anything in Latin America.
Check out this thread to get you an idea of logistics for central american borders: http://advrider.com/forums/showthread.php?t=141751
Dont stress too much about the planning, you will figure it all out as you go along.