If what you're looking for is a brass rod with a ball on the end - a muzzleloading "short starter" should work fine. Many are threaded on the end to attach a bore jag to for starting patched round ball loads and others have a concave end on the rod. The shaft diameter is determined by the caliber of rifle you are shooting. I have several in my shooting box for various calibers and that's what I'll go to for a squib if I ever end up with one. Normally the shaft on the short starter would be long enough for most barrels 6" or under - depends on the short starter - but by putting a brass jag on the end, you can extend it a little further.
Check on eBay for "short starter" or "bullet starter black powder", etc. - lot's of times there are uses ones on there that can be bought cheaply or check out a LGS and see what they have in their muzzleloading supplies. Some of the short starters I have were made up a number of years ago for me by a fellow - I have several in 5/16" for smaller calibers and several in 3/8" for larger - they have a larger diameter round brass T handle on them. I had mine made with a concave end and then threaded for a screw on cleaning jag. I have another with a nice sized ball handle. Something like that should do the trick.