I dropped my laptop a while ago, and the plastic casing on the bottom cracked, and the crack is getting larger. Is it possible to get a replacement part and replace the casing? Where would I be able to find the replacement case as well?
eBay is a good place to find plastics for laptops. It's a bit of work swapping all the guys to a new case, but if you're confident doing it, you can usually get lower cases pretty cheap. You didn't specify a make / model, but if you search on eBay for parts for your specific unit, you most likely will find one.