You could always keep the original drive in an external USB drive. You can get one of those external cases for about $20.
If you're willing to give up your DVD drive (at least part of the time). You can get an optical bay caddy that lets you swap out a 2nd HDD/SSD with your DVD unit. Optical Bay Caddy
External DVD drives aren't too expensive if you don't like the idea of swapping hardware.
There are notebooks that offer 2nd HDD/SSD as an OEM option - HP Pro Book would be one example. You see these options more often in the Business class notebooks.
You'd have to look in the larger, higher end laptops to find one with two actual drive bays.