It must be evident for you that digital music distributors predominantly require digital audio premasters guaranteeing ‘CD quality’. By that they mean 16-bit WAV files. Their sample rate is44.1 kHz. It won’t be difficult to get such premasters especially if the upcoming project is released on CD. As a matter of fact, your online mastering services engineer uses similar digital audio assets when he gets the CD duplication master ready. You’ll have to require premasters as a separate file set beforehand instead of ripping them from a CD in the future.
Music distribution is now available online, and it’s not difficult to find the information on delivery specifications in the Internet especially if the mastering engineer you’re working with is a qualified expert. Take into account that album art and metadata as non-audio deliverables can definitely have some extra requirements and specifications. They’re all pruned down by the use of an aggregator and are also available online.