A couple vendors which stand out to me (which
@Victoria has already mentioned):
Lazy Cat Tea http://www.lazycattea.com/
I've tried a few of their teas -- their "standard" Rougui, a zhengyan Rougui, and another zhengyan tea with the lovely designation "Space Cat Rougui."
The first is very much a budget tea (13.00€ / 50g), and I didn't expect to enjoy it. I was surprised however -- although the underlying material is obviously not Super High Quality (and Lazy Cat doesn't make any claims to that effect), the tea is well processed for what it is. It lacks durability, but the roast is well done, the flavors are well integrated, there is a pleasant aftertaste. It tastes clean. There are little/no off flavors. It doesn't have much of the qualities which make yancha special, but it is recognizably yancha.
The two zhengyan teas are much better, with those special yancha qualities coming through more clearly. Again, the processing is very well done, and these are well balanced teas. The Space Cat is particularly good, with nice fruit/peach flavors which are well integrated. I wish I had more of the Space Cat (I only got 50g and it's sold out

), as it's clearly going to age well and into something delicious.
Essence of Tea https://essenceoftea.com/
I've been regularly buying tea from EoT for the past 5 or 6 years. Their 2018 Lao Cong Shui Xian is/was in my opinion a very good value ($0.85/g), especially with the tea club discount of 20% (which put this tea at $0.68/g). I only got around 400g at the $0.68/g price, and it's unfortunately sold out now. With the discount, I can't think of another yancha in recent memory with a comparable price/value ratio.
I've also had from EoT the Lao Shou Xin (this was $4/g for a sample, $3.70/g for 100g -- sold out now), and a 90's HK-style Shui Xian (forget the exact price, also this came in the tea club, but well over $1/g). The former is in my opinion a benchmark tea, and the latter an exceptional example of aged yancha.
My impression is that both of these vendors know what to look for in yancha, and are trying to provide a good quality/value ratio in their selections. They're both high up on my radar.
Another vendor whose yancha I've enjoyed is
Chanting Pines. Of the yancha I've tried, the Bu Zhi Chun stands out as really excellent. (I don't know if I should say that too loudly, as it's not sold out yet and you might've noticed a... pattern

).
Other vendors whose yancha I haven't tried, but would like to as they seem interesting/promising:
The Cha Tong https://thechatong.com/collections/yancha
Tong Xin She https://txs-tea.com/collections/yan-cha
@Bok's point about the price scale is true, and well worth keeping in mind. One just is going to pay more in general for yancha than for other teas of comparable quality.
Hmm wrote: ↑Sun Apr 05, 2020 9:21 pm
Where are the diminishing returns when it comes to e.g. Yancha or any other teas?
This is a good question, and one I wish I could answer for myself, but can't yet with yancha. For me, the trouble is -- I can think of examples of other particular teas (I'm considering mainly puerh, liubao) which have no real substantial flaws, are excellent to drink etc but which cost well below $1/gram. Like, it's not just that yancha is on average more expensive, it's that, for me at least, it's easier to think about diminishing returns when I have a particular benchmark tea in mind whose price/quality ratio is exemplary.
Thinking about all this leaves me feeling I should spend the next few weeks focusing my tea drinking only on yancha...