"Make" one.
My first notebook (HP650) had a single 2.5 inch HDD. I bought a cheap HDD caddy from eBay, removed the DVDRW and replaced it with the caddy and a second HDD.
Later I saw that there are also DVD caddies. So i got one and installed the DVDRW, works with USB2, and it's good enough. I did not use it much but it's working fine.
Both caddies cost around 15$ (5-10$ each). Got an old laptop for spares?