The only way to get Cobra Skins is from a Serpentarium. Not a lot is known about the way the Cobras shed their skins, other than the fact that the more well fed and generally happy Cobras you have, the more Cobra Skins you are likely to get. Simply check your Serpentarium each day for Cobra Skins and take them. Taking the skins does not seem to disturb the Cobras in any way. If you leave the Cobra Skins in the Serpentarium they will not get destroyed, so failing to check for Cobra Skins for a week is not a problem.