What ways can I purchase a newspaper subscription for viewing on an iPad?
For the Kindle system, if one looks carefully under the product description such as you would find under this list, http://www.amazon.com/Kindle-Newspapers/b?ie=UTF8&amp;node=165389011,  one can see a link for "Available on these devices". This link will tell you if an Amazon subscription for that newspaper can be read on "Free Kindle Reading Apps",  or exclusively on "Kindle e-readers".  

In order to have the freedom to read on multiple devices (which seems to be the intent of your question, but forgive if I assume too much), you'd want to choose a newspaper available on "Free Kindle Reading Apps".