Store4PC via Amazon offers Prime members the Sabrent 2.5"-to-3.5" Internal Hard Drive Mounting Kit, model no. BK-HDDH, for $6.49. Coupon code "2SSDBRKT" drops it to $4.22. Plus, Prime members bag free shipping. (Non-members can spend $49 or more to qualify for free shipping; otherwise, shipping costs $8.16.) That's the lowest total price we could find by $3. It supports any computer case with an available 3.5" drive bay and includes mounting hardware.
Also available for Prime members is the Sabrent SSD / SATA Hard Drive Connection Kit, model no. BK-HDCC for $5.84 via the above coupon with free shipping. (Non-members can spend $49 to qualify for free shipping; otherwise, shipping costs $8.19.) That's $17 off list and the lowest total price we could find. This kit includes a mounting bracket and four Serial ATA cables.