For Thirumandiram and other thirumurais like thevaram - refer
www.thevaaram.org
For many sangam and other tamil works - you can buy from Then India Saiva Siddantha Nool Pathippuk Kazhagam, Thirunelveli. ( they have a branch in Chennai also)
The South India Saiva Siddhanta Works Publishing Society LtdThe South India Saiva Siddhanta Works Publishing Society Ltds=4202535429552991917Place page522, TTK Rd, Alwarpet,, AlwarpetChennai, India., Tamil Nadu 600018- 09884684666 Open Mon-Sat 10am-5:30pm