Have you tried the NOAA site for online updates?
Notice to Mariners Chart Updates
type in the number of the chart you wish to update and press get chart. You are given the options on the next screen
to order a new updated paper chart, view the chart on line, or download the latest Rastor chart. You can also check the Notice Listing which will give you not only the list of changes, but also the date of the update publication.
For non US territorial waters, you will likely have to go the the hydrographic offices of the countries you plan to visit. Do you have an electronic chart plotter of some sort? If you do you can down load the electronic charts from the vender or manufacturer site.