You can buy them at various places online, but ebay seems to have a decent range and some great prices too!
The more straps a suspender belt has the more stockings will be held up evenly. 6 is a good number – but they go up to 12 or even 14 straps!
You can buy suspender belts with 6,8,10,12, and 14 Straps!
12 Strap Suspender belts at ebay