I bought an AT 3600 online from revolver club last year https://www.therevolverclub.com/collections/accessories but it seems to be sold out as of now. You can try calling them and asking if they can get one for you or you can try doing a Google search to see if there are other websites in India where you can buy it or other cartridges online.
Get it from 123sold.in(New Gramophone House) for ₹5000. You can also get AT VM95C under ₹5000 from Amazon. Com. It is upgraded version of 3600l. Also try for AT91R. Your tt is a gem, it can compete with any expensive one.