Prices range between $500 (£360) and $750 for doses of AstraZeneca, Sputnik, Sinopharm or Johnson & Johnson jabs.
Fake vaccination certificates are also being sold by anonymous traders for as little as $150.
Researchers say they have seen a "sharp increase" in vaccine-related darknet adverts, while the BBC has been unable to verify if the vaccines are real.
The darknet, also known as the dark web, is a portion of the internet that is only accessible through specific browser tools.
Researchers at cyber-security company Check Point have monitored hacking forums and other marketplaces since January, when vaccine adverts first appeared.
They say the number of adverts they have seen has tripled to more than 1,200.
Sellers of vaccines appear to be from the US, UK, Spain, Germany, France and Russia.
The team found multiple adverts in Russian cyrillic text as well as in English.
The vaccines advertised include the Oxford-AstraZeneca at $500, and Johnson & Johnson and Sputnik each at $600, and Sinopharm at $750.
One seller is offering next-day delivery, saying: "For overnight delivery/emergency leave us a message."'
Another advert on a hacking forum is offering fake negative tests and reads: "We do negative Covid tests, for travellers abroad, for getting a job etc. Buy two negative tests and get the third for free!"
Some holiday operators require vaccination certificates for passengers.
A vaccine passport system is also being considered in the UK and could be used to allow visitors entry to venues such as bars, or sports stadiums.
European officials have also announced plans for a "Green Digital Certificate". This would allow anyone vaccinated against Covid, or who has tested negative, or recently recovered from the virus, to travel within the EU.
It's no surprise then that faked documents are being offered on the darknet for sale.
Check Point investigators found many sellers offering forged documents, including one supposedly from the UK, with a vaccination card for $150 using the hard-to-trace cryptocurrency Bitcoin as the payment method.
When they got in touch with the seller, the team were told that they just needed to provide their names and some dates for when the fake jabs occurred. The seller messaged: "You don't have to worry…it's our job….we have done this to many people and it's all good."
Oded Vanunu, head of product vulnerabilities research at Check Point said: "It's imperative for people to understand that attempting to obtain a vaccine, a vaccination card or negative Covid-19 test result by unofficial means is extremely risky, as hackers are more interested in your money, information and identity for exploitation."
Mr Vanunu also told the BBC that his team purchased a dose of the Sinopharm vaccine from a vendor for $750 as part of their research, but are yet to receive it.
His team has told the BBC that they believe this seller was a scammer, but say others may or may not be selling real vaccines.
Check Point is urging countries to adopt a QR code system across all vaccine documentation to make forgeries more difficult.