Konstantin:

Try http://www.woodfinishingsupplies.com/
click on Shellac link.
This site has about 2 hours of reading on shellac and the products they
sell.

As you said, you purchased shellac from your local hardware store, but since
not all shellac is created equal (natural product like wine, beer, etc) I
suggest you try another source. Several sites on web sell high quality
shellac, and with higher volume sales, products from these site may not have
been on store shelf for long time.

Phil