Glad you asked...
Semtex can only be legally manufactured by blasting companies and their suppliers. Illegal production is a felony and suspected materials must be handled by a bomb squad.
Then again, this answer might help you...
Semtex is a very powerful explosive made of plastic that is used in industrial environments. A mixture of RDX and motor oil will create the consistency of this substance. For more information look here: en.wikipedia.org/wiki/Semtex;

