Below are some good internet sources for buying CDs and other items related to Carnatic music. Most of the sites will offer an internet mail order service. I do not endorse any of the products or services.

Music CDs
Charsur Digital Workstation (Carnatic CDs. Chennai)
Rajalakshmi Audio (Carnatic CDs. Coimbatore)
Samskriti (Carnatic CDs, Books, etc. Chennai)
Raag Online Music Store (Indian music CDs. LA)
Sa Re Ga Ma (Indian classical music CDs.)
Khazana (Indian classical music CDs, books, paintings. Minnesota.)
Peshkar Indian Music (Hindustani music CDs. Germany.)
Oriental Records (Indian Classical Music CDs. New York.)
Koel Music (Hindustani and Carnatic CDs.  Pune)
SonicSoul Acoustics (Karakudi Veena Tradition CDs)
Natya Store (Carnatic music DVDs and CDs)
 

Indian Musical Instruments
Sapthaswara Musicals (Indian Musical Instruments.  Chennai)
Musee Musicals (Indian Musical Instruments.  Chennai)
House of Raga (Indian Musical Instruments.  Canada)
 

Indian Music Books
Karnatic Music Book Centre (Huge range of music and dance books.  Chennai)
CBH Publications (Books on music and dance.  Nagercoil)
Vedams Books (Large range of Indian books)
Kalã Arts Quarterly (A South Asian Arts magazine.  Canada)
 

Indian Dance Supplies
Shanthi's Dance Needs (dance costumes and jewellery items. Chennai)
Natya Store (Dance needs, Huge range of Bharatanatyam and other dance DVDs etc)
Bharatanatyam World (Dance needs)
 

Software
Rasika and Gaayaka (Carnatic Music Software for PC's)
Animated CD Rom on Thyagaraja (by Solfa Creations)
Radel India (Electronic Music Instruments like Tanpuras)