Prices range between $ 500 (£ 360) and $ 750 doses of AstraZeneca, Sputnik, Sinopharm or Johnson & Johnson jabs.
Counterfeit vaccination certificates are also sold by unknown traders for as little as $ 150.
Investigators say they have seen a "significant increase" in darknet-related vaccine ads, while the BBC has failed to confirm whether the vaccines actually exist.
Darknet, also known as the dark web, is the only part of the Internet that is accessible only through certain browser tools.
Investigators at cyber security company Check Point have been looking at hacking forums and other markets since January, when vaccine advertising began.
They say that the number of ads they have seen has tripled to over 1,200.
Vaccine sellers appear to be from the US, UK, Spain, Germany, France and Russia.
The team received many advertisements in Russian and English cyrillic text.
The advertised vaccines include Oxford-AstraZeneca for $ 500, Johnson & Johnson and Sputnik each for $ 600, and Sinopharm for $ 750.
One vendor offers the next day delivery, saying: "Night delivery / emergency leave a message." '
'Buy two and get one for free'
Another ad on the hacking forum provides fake tests and it reads: "We do bad Covid tests, for overseas travelers, get a job etc. Buy two bad tests and get a third for free!"
Some holiday operators need passenger vaccination certificates.
The vaccination passport system is also considered in the UK and can be used to allow visitors to enter places such as bars, or stadiums.
European officials have also announced plans for a "Green Digital Certificate". This will allow anyone who has been vaccinated against Covid, or who has been found to be HIV-negative, or recently infected, to travel to the EU.
It is not surprising, then, that counterfeit documents are offered on darknet for sale.
Check Point investigators have found many merchants offering counterfeit documents, including one assumed to be from the UK, and a $ 150 vaccination card using the hard-trace cryptocurrency Bitcoin as a payment method.
While contacting the dealer, the group was told that they only needed to provide their names and specific dates for the fake jabs. The salesman sent a message: "You don't have to worry… it's our job…. We've done this for a lot of people and everything is fine."
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Oded Vanunu, head of product risk research at Check Point, said: "It is important for people to understand that trying to get a vaccine, vaccine card or Covid-19 negative test results illegally is very dangerous, because hackers want your money, details and and ownership of the exploitation. "
Mr Vanunu also told the BBC that his team had bought the drug Syopharm vaccine from a retailer for $ 750 as part of their research, but they would still get it.
His team told the BBC they believed the dealer was a scam, but said some could sell real targets or not.
Check Point urges countries to use the QR code system in all vaccine documents to make frauds more difficult.