The best place to find detailed, in-depth histories of the countries you're researching is in the books in our library's collection. So, if you were comparing and contrasting the governments of Poland and the Czech Republic, you might want to start by looking at the political history of those two countries.
You could try the following searches in the Bentley Library Catalog below:
Poland AND history
Czech AND history
Czechoslovakia AND history
These searches bring up a number of books on the histories of these countries that would provide invaluable information for a paper comparing the two countries' political systems.
You could replicate these searches in the WorldCat Catalog to look for books outside the library's collection on these countries. If you find some books that we don't have in WorldCat that would be useful to you, be sure to request them through Interlibrary Loan.