They have it on some hobby sites...

http://www.luckymodel.com/scale.aspx?search=Y&q_brand=&q_category=&q_scale=&q_word=perfect%20plastic%20putty&q_show_instock_only=N

http://www.megahobby.com/perfectplasticputtydeluxematerials.aspx

Or search it on Amazon. Plenty of listings there as well.