One of the agents selling the professionally made cards said his network had sold 1.5 million cards over the past three months.
It is believed that there are many such networks selling the fraudulent cards.
About five million people are expected to vote in Saturday's elections. If election observers fail to detect these fake cards, the millions of fraudulent ballots in circulation could have a significant, maybe even decisive impact, on the whole election.
In Jalalabad, a city east of Kabul, Farooq Millanay, the local member of parliament, is so concerned about the effect of these cards on his own ballot that he is calling for the polls to be postponed.
Jalalabad appears to be at the centre of the fake-cards market.
Last year's elections in Afghanistan were marred by claims of fraud and corruption. Complaints of vote rigging and irregularities plagued the process, which yielded a questionable verdict for the incumbent president, Hamid Karzai.
In the course of its investigation, Afghanistan's Electoral Complaint Commission, backed by the United Nations, found "clear and convincing evidence of fraud" in certain provinces.
Karzai's campaign spokesman dismissed the allegations of fraud as "rumours".