Check out IRD links for NBT & VAT
http://www.ird.gov.lk/en/Type%20of%20Taxes/SitePages/Nation%20Building%20Tax(NBT).aspx?menuid=1206
http://www.ird.gov.lk/en/Type%20of%20Taxes/SitePages/Value%20Added%20Tax%20(VAT).aspx?menuid=1204
You can read the Acts with latest amendments
Also check out for any changes made through Gazatte if any
http://www.ird.gov.lk/en/publications/sitepages/Gazette.aspx?menuid=1602