Replacements for a 4006 IC?
Don't listen to these RadioShack guys, giving such advise is a shame to the shop!

The alternative devices you mention are entirely different. The 4000-series are digital devices and 4006 specifically is a digital shift register, both LM's are analog devices called comparator and opamp repectively. These 4006 chips are pretty standard and shouldn't be too hard to find in a decent electronics shop.