<p>Try looking on the dover publications site. I’ve found some of their books to be good and they are very cheap too. [Differential</a> and Integral Equations](<a href=“http://store.doverpublications.com/by-subject-science-and-mathematics-mathematics-differential-and-integral-equations.html]Differential”>http://store.doverpublications.com/by-subject-science-and-mathematics-mathematics-differential-and-integral-equations.html)</p>
<p>Check the reviews on amazon before you buy though. I didn’t for one of the books I bought, and although it has almost all 5 star reviews it is definitely not an introductory book, the reviews will mention this kind of stuff usually.</p>