How to change AppStore for my iphone4 & ipad2?

You can only buy content from your own country's store - to buy from another country you need a valid address in that country, if using a credit card then that needs to be registered at that same address, and you need to be in that country when making any purchases.

You can try requesting that the apps be added to your country's store, but ultimately it's the developer that has to make it available to Apple to sell in a particular country (and sometimes a country has laws that restrict what can be sold) : http://www.apple.com/feedback/itunes.html

If you have a valid address in that country and you will be in that country when purchasing.