There isn't a standard go-to for any of these really sadly.
I know a couple of guys have had very good luck finding a good match to the Season 15/16 waistcoat via ebay and I feel this isn't too tough of a fabric to find a reasonable match to.
The Season 18 Vest is very problematic due to it's cut, the design and the fact it's a silk velvet. The best replica that nearly made it was the Cloth ears but that ended up being DOA when they left the Who Costuming game before making any beyond the protoype.
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Next best matches are usually found on western recreation wear sites but you're settling for something with the vague feel - usually on sites like gentlemansemporum.com or a few others that have been discussed on Gallifrey Base:
Gallifrey Base
A few people have found some fabrics that are semi in the range- if you find something that works for you then you can have it easily mad einto a vest via a local tailor or any of the usual online suspects.
The S13 Houndstooth check waistcoat is another one that a lot of people have found okay matches to at thrift shops and careful ebay shopping but usually sacrificing the houndstooth for the check or vice versa usually a striped tweed waistcoat of some type.
But recreating this S13/14 vest fabric has been a project I've worked on for a long while now. Thanks to greyhound1 on GB we're working with a pattern I'm happy with but to do this on velvet (like the original was) as a screen print process will require a 25 minimum person (or yards accounted for) run at about $65 or so a yard. I wanted to wait til my current group runs were finished before pursuing this in it's own thread. Keep on the lookout for the thread wen it gets going.