Magnetic therapy is a worthless quack treatment for arthritis. Magnetic bracelets and shoe insoles are sold for this purpose. If you absolutely must have one you can find it — where else? — at your local health food store or New Age bookstore, or advertised in the Weekly World News.
The general idea is that magnetic fields work miracles such as "increase circulation" or "reduce swelling". While pulsed, high powered fields have been shown to have some benefit on some wounds, the effect of exposure to very small magnetic fields is negligible to zero. Hæmoglobin, for example is slightly diamagnetic, but it would take extremely powerful electromagnets or superconducting magnets to show any influence on blood flow, let alone significant or positive effect — and as MRI scanners are considered to be pretty safe, and are far stronger magnets than are available to magnetic therapists, there isn't really that much of an effect.
Most studies done on magnetic fields have yielded the result that wearing magnets has no effect above a basic placebo. For example, the pain due to arthritis — a joint condition sometimes targeted by magnetic therapy — varies from day to day or even from hour to hour; as a result, patients can find it hard to tell whether the treatment is actually working. Selective reporting and confirmation bias takes care of the rest to ensure that any scam treatments get a positive reception.