Basic Comfort makes the Ultimate Crib Sheet (web: basic-comfort.com). It sells for $20 and is available at Babies R Us, Target and other baby web sites..
Note: I found this harder to use than just replacing the sheet on my latex mattress since the mattress is so light.
How many diapers do you need for starters? Are you sitting down? If you're going with disposables, we recommend 600 diapers for the first six weeks (about 14 diapers a day). Yes, that's six packages of 100 diapers each (purchase them in your eighth month of pregnancy, just in case Junior arrives early).
Note: G diapers tiny size worked really well too
If you plan to use a diaper service to supply cloth diapers, sign up in your eighth month.
Note: My favorite cloth diaper is the Bummis Easy Fit. Also a favorite: FuzziBunz. What's nice about these is if you get the one size, they last from about 1 month old through toddler.
For first time parents buying for a newborn, we recommend you buy “3-6 month” sizes (instead of newborn sizes).
Note: I'd wait until the baby showers are done. You often get all the clothes you'll need for those first 3 months from your shower.