Hi Tall Paul
If you are intending to sail your model I suggest you visit
http://www.pjsails.co.uk/ and download his catalogue. if you scroll to the end you will find turnbuckle and swivels and rigging cord. Mainly designed for the model yacht fraternity so may be a bit heavy for your purpose.
However you can then visit other model suppliers, as already suggested, and look for the correct parts.
If you do choose shirring elastic you will need to replace every couple of years. I have a large 4' Clyde Puffer and use shirring elastic for the standing rigging as it allows me to drop the mast for storage. The running rigging for the boom, hoist etc is standard rigging cord with home made fittings, but you can get these from Cornwall Model Boats and others
Good luck
Dave