If you go on eBay (which is where your cue came from originally) and do a search for "5/16x18 shaft" (you can copy and paste that if it's easier) you'll get options that range from less than $30, to over $500 for the carbon fiber shafts. Read the descriptions for the size of the tip and the taper profile of the shaft, and pick what you want.
The butt of your cue, being already machined to accept a piloted shaft, means that either piloted or flat faced will work for you. The diameter match at the face of each piece may be slightly off, but only by a few thousandths, and you'll forget about that quickly since your stroke never involves contact with that area.
I'm sure that if you don't want to keep the original shaft you have, that you could sell it for around $50 with no problem at all.