As far as I know, thiopental is not subject to international restrictions, unlike most of the barbs. I haven't seen it listed in any United Nations controlled substances table.
Your country may apply some restrictions though, so you'd better check your local government regulations.
As you said, there are legit sellers on IndiaMart.
This post may help you in assessing whether the seller is likely legit or a scam.
Be aware that some scammers harvest data such as addresses, employee names and logos from real, legit companies but use their own phone number, so it is imperative to check all data before buying. But that's common sense.
If you fail to source it from IM, I might have a legit source for you.
By the way, can dexmedetomidine be consumed orally for CTB?