Unfortunately Thunderbolt cables use electronic boards within the connectors, which has prevented third party manufacturers from making cables for just a few dollars, like what is common with USB. Your best bet is to find a spare one that someone is selling on eBay or somewhere else used.
There is no difference between thunderbolt I and II cables. Thunderbolt II adds the ability to combine the two 10 Gb/s lanes into a single 20 Gb/s lane.