I have a Intel Pentium 4 CPU, which was used to mine bitcoins in 2009*, (I think this kinda cpu is the oldest mining hardware) and you can find it on amazon or ebay just for couple of bucks.
Ps: I wasn't mining back then but my computer did, I had a virus called "minerd.exe"** which used my cpu 100%, (I had to reinstall Windows XP, whenever it infected me).
** Yes I know it's not a virus right now