Hi 262jennifer. Welcome to MR!
Sorry I can't help with mobipocket questions, but there are a LOT of nicely-formatted (and legal) books that you can download from right here on MR. Choose mobipocket format (.prc) files.
There are also a lot of very knowledgeable and friendly people hanging out around here. You'll get lots of advice and suggestions. If you don't get the answers you need here in "Introduce Yourself", try one of the more technical forums, like
https://www.mobileread.com/forums/forumdisplay.php?f=140