newegg offers the Rosewill 3.5" Serial ATA Hard Drive to 5.25" Drive Bay Mounting Kit
, model no. RDRD-11006, for $3.99. Coupon code "BTEJJHB64" cuts it to $3.19
. With free shipping
, that's $2 under the lowest total price we could find for a similar mount elsewhere. It allows you to mount a 3.5" SATA internal hard drive in a 5.25" drive bay.