You can buy new repro tanks on eBay from India, see This Example
, however the quality is often poor.
I once bought an unpainted replica from India, internally it was full of rust and the fit on the bike wasnt great so adjustments needed to be made. I removed the rust and sealed the tank internally then had it painted and now it looks good.
Although you can buy painted tanks from India I feel its better to buy an unpainted one as adjustments will invariably be needed.
I know others on this forum have had good experiences with Indian tanks so hopefully they will chime in with recommended sellers on eBay.