There were several 30GB iPods, from 3rd gen to 5th gen. Is this a "video" iPod (5th gen)?
If it's 3rd or 4th gen, it will definitely NOT fit. If it's 5th gen, it probably will not fit, unless (maybe) it is really rubbery and flexible, or loose fitting. The top edges on the new iPod "classic" are more rounded compared to the 5th gen iPod, although the overall dimensions are similar.
There seem to be a lot listings on eBay, if it's 5th gen. Do a search on "ipod classic 5th gen case." Pretty cheap, because they are old products. NOTE: There were two "thicknesses" for iPod 5th gen, depending on capacity of the hard drive. 30GB is the thinner type (for a 5th gen). If the case is a hard shell, be sure you get the correct match.