An anticholinergic product that used to be sold was Calmitol, based on hyoscyamine.
If you're really desperate and at the same time ambitious and a bit of an herbalist, you could look for some plants that produce belladonna alkaloids and try to reproduce something like Calmitol. Herb shops probably won't sell something that toxic, but there are plenty of wild plants that do produce them, not just belladonna.
As an Herbalist I need to respond to this.
WARNING!!! Do not go looking for belladonna or belladonna like alkaloids to alleviate bug bites. Or anything else even if you consider yourself 'a bit' of an Herbalist.
The alkaloids in Belladonna and Belladonna like alkaloids are
TOXIC! Yes, they are synthesized, standardized and used commonly in medicine - by TRAINED PHYSICIANS.
The alkaloids in Belladonna and Belladonna like alkaloids cross the blood brain barrier, they affect the central nervous system, they can affect your heart, they can put you in a coma, they can kill you.
If you have any interest in using Belladonna or Belladonna like alkaloids, go see a Cardiologist or GI specialist. If you want to try using Belladonna or Belladonna like alkaloids for any reason do it in an Emergency Room with trained Physicians available and aware of what you are doing before you do it. They will stop you.
Hycosciamine was used in Calmitol at 0.0001% and it was taken off the market.
http://drugs-about.com/drugs-c/calmitol-itching-relief.html
Even if you are desperate, stick with other herbs. Plantain and calendula are wonderul for skin irritations -as long as you are not allergic- as are many others herbs, but please do your homework before using and picking any herbs that you are not familiar with.
Edit: Belladonna is commonly called Deadly Nightshade. There is a reason for this.