If you cannot find the Search provider you want in your country's
Internet Explorer Gallery Add-ons site try going to another country's site.
Yesterday I tried to add the Google search provider , I live in the United Kingdom so my default is Internet Explorer Gallery
but there is no Google search listed ?
To get less options change the country code (its the 2 letters after com/ ) to au thats the Australian site they only have 3 listed.
for more change to ru Russia, cn China, us United states ect...
Seems not all country's are supported but plenty are.
You can find country codes at ISO 3166-1 alpha-2 - Wikipedia, the free encyclopedia
Another way of of getting more search providers is use the Opera browser very easy to add the search from any site.
edit: Its just as easy to expand the choice of other categories of add-ons
Just found a easy way to make your own Searches
"Add your own search provider to your copy of Internet Explorer by following these steps:
Visit the desired search engine in another window or tab
Use the search engine to search for TEST (all capital letters)
Paste the URL of the Search results page"
into the box on this page Add Search Providers to Internet Explorer