When it comes to optical lenses of any kind, you can buy very cheap or very expensive.
To a degree, you can get what you pay for. A very cheap lens will distort around the edges, and these cause eye strain after a while.
A very expensive lens *might* have better optice (but in this case you might pay a premium for the frame and/or age- if it's an antique).
If you plan on using it for long periods of time (say an hour or more) I wouldn't suggest getting anything too cheap, as it can give you ha headache! But I also wouldn't go overboard on the quality- there is a certain point at which you get diminishing returns. This point will be different for different people (those with bad eyes will have more trouble seeing the difference).
I would suggest something 3-4 inches in diameter, and not made with a plastic lens. A pretty good one might cost as much as a normal tarot deck would cost. If you get smaller lenses, you will find them harder to work with because the distorted portion on the edge of the lense is so close to the area you qill be focusing on.
Another alternative is something that sits on a table top, and is designed to focus on the tabletop- this keeps you from having to move the glass back and forth to focus it yourself- but this also means that you will have to spend your time hunched over the table. If your back is as bad as mine, this is not the best option.
Edit to add- I wouldn't necessarily go for the high magnification jewelers glasses. Because most tarot cards are printed on modern printing presses, if you get too close, you will start to see nothing but a collection of little cyan, magenta, yellow and black dots.
-tb