Has it gotta be new? Folks regularly toss jackets up for sale in the 'Wanted' section. There might be one or two there now. Sure, I adore my elderly Belstaff Trialmaster that I got scarcely worn off a remote Craigslist in Colorado, where it had turned up in a Google Search ages ago, for $100 shipped along with some matching trousers that also fit well, though I seldom actually wear them, as nowadays I'm a bit of a "Fair Weather Rider". Yet that didn't stop me pouncing on an well-worn older style waxed cotton Aerostich "Aero Darien" style wonder jacket (very similar to their current "Falstaff" model), which our man 'olhogrider' was shedding. It needed a good hosing down and soaking in Woolite, a very light detergent usually used for woolens and delicates, to get a fair portion of the New Mexican Desert and a couple-few dazed scorpions out of it, a little ultra-fancy embroidering-grade needlework by my Olde School Euro Trophy Wife to patch a little dime-sized hole in the sleeve, followed by a thorough wax reproofing. But MAN it's nice, and nicely fills that climate gap with its many zippery and velcro breathing farkles between the decidedly cool weather Belstaff and some Bike Bandit clearance mesh rags I wear when that mercury's a-poppin'. Hell, after re-proofing that "Aero-Darien's" color even matches my Military Bullet's. Thanks again, 'olhogrider'!
Long term I've been OK with waxed cotton. It's hardly been the chore for me to own that 'glencoeman' describes. I merely brush 'em down and touch up or re-proof with the waxy goo and a heat gun at most every couple of years or so, and rather enjoy the process. But then I ain't hauling my Bullet in and out of marshes regularly or anything.
You might get lucky with something lightly used here too. But for the love of God, whether you choose to go the Old Clobber (i.e. preowned ridingwear) Way, are reaching out about parts new or used, or anything like that, please take a moment to bang out your Profile so we'll know if you're in the UK, USA, Tasmania, or one of the more eccentric moons of Neptune, OK? A bargain here may not be a bargain there after shipping, and availability or likely parts sources can vary geographically too. Doing it saves everyone time.
And if you choose to go the factory fresh route, and are leaning towards waxed cotton, do check out that current Aerostich "Falstaff":
https://www.aerostich.com/suits/falstaff/standard/falstaff-jacket.html. Sure, they ain't cheap. $700 is a bit of a splurge for many, but it's real quality for money, bespoke and handmade in the USA, and with quite a few more tricks quite literally up its sleeves for safety and comfort than the traditional Belstaff, though for roughly the same price. One could do worse. A fuzzy look at my hand-me-down predecessor with largely-unearned "patina" is attached. Trust me, it definitely looks more "dashing and adventurous" with someone in it...a kind of rumpled yet gentlemanly "No, I couldn't eat a bite--just had dinner with some Bedouins in the desert" vibe.