These are just some of the things I've done but I give-up intermittently. After my last 'grail' (6 years of tracking one down) I just figure other scarves on my list will find me - sometime.
1. Let it be known (so long as it's not on the general HG list). I've had tPFers notify me if they seen 'x' on sale at Y; 1000 of eyes are better than one. Forget this if it's a grail for many, as resellers read tPF and spiral the prices.
2. Go to all the usual haunts online, e-bay, Vestaire etc. Repeat. Repeat. Repeat. Some of these sellers won't know titles, artists (even if they're written on the scarf) cws or very much at all so you have to wade through and can't rely on word-searches. I don't have a PayPal account which makes my hunt more difficult (but not impossible).
3 Google the title + Hermes and go to 'images'. Hopefully your scarf will be there, though it usually just takes yo back to tPF.
4. Be patient and be picky and wait until the heat's off. It'll be the day you're not looking that one falls from heaven straight into you lap. Don't buy an OK cw or similar design when it's not the one you really want as you'll still be after your original grail. Don't buy a grail in a 'bullish' market, fashions change.
5. Buy the reissue. I never thought I'd get a Paddock in a cw I liked (there was precisely one cw I liked from past issues) and then H issued so many gorgeous cws it was hard to choose.
Excellent advice. I agree wholeheartedly.