I’m not sure what these things are called, but they are waterproof trousers you can wear over your trousers (not the cycling kind which also have covers for feet).
I used to have one which had zippers from the waistband on each side to divide it, but these were recently lost on a trip.
Anyone have a recommendation on where to buy something like this again (I previously ordered online via wiggle.co.uk which I think is now gone) and what search terms to use?
“Regenhose Männer” did it for me.
I picked the top search result which happened to be Ochsner.
OH has some he will not part with for love nor money…NASA style insulation, outer shell Gortex. 400€ 20 years ago, with a matching jacket.
Wiggle, along with many other bike online and off-line retailers when bust after COVID, mostly from having too much stock and no interest from buyers.
The name was bought by Sports Direct and trades now with lower-priced/quality items in a similar style as Sports Direct.
They do ship here and occasionally have some bargains:
More choice than Decathlon too.
While on a hiking holiday in Switzerland, I walked by a shop which advertised 50% off on rain trousers. So I thought, why not get it now. Then I looked at the price: 550 CHF after discount! 
yep, they’re not cheap. The ones that will allow you to enjoy solitude and quietness – (because otherwise, the sound of “plastic frottage” will get to your nerves after 10 minutes walking)-- start at that price. If you don’t care so much and/or are happy with stiffer / plasticky / heavier over-trousers, you can get some for 200 - 300 CHF
Otherwise, you always have ‘Ricardo’ 
p.s. i don’t recommend buying on Temu or internet – low quality, easily ripped off, or a hot-sauna (i.e. not breathable) are the most common problems.
My OH is having difficulty these days finding the type of cargo trousers with zip off legs that turn them into shorts. He has a decent navy blue pair from years ago, but managed to rip the back of them on his bike. I managed to do a repair but it’s not great. They used to be freely available before the pandemic.
I don’t know what fancy thing your looking for as for me this would only be something I’d wear if surprised by a rainshower and also not for hours on end.
Like ShirleyNot I’d go for something simple and I have yet to buy something bad from Landi 
I think my previous ones were made by Berghaus.
Thanks for the Landi tip. It’s pointless for me to get something posh because they are mainly for wet dog walks so get splashed and covered in smears of mud and leaves so Landi looks like a winner.
@Phil_MCR - you could buy Berghaus waterproof trousers from other outlets.
I bought those from Landi some years ago and they are fine
I was in Aldi Germany last week and they had a lot but cannot guarantee they are still there
Another thumbs-up for Landi Regenhosen.
A torrential downpour is no excuse to skip Robin Goodfellow’s morning constitutional; Landi trousers (and jackets) keep me dry. A Muttley walk is more often than not a muddy walk, these clean up easily. And the price can’t be beat. Expensive outdoor clothing makes no sense for my kind of dog walking.
Only downside is that the trousers come in a standard length, which is too long for me. But they are easy to hem.
How much… can I interessant you in my bin bag and gaffatape autumn/winter Collection?
I had a Berghaus waterproof jacket which wore out so I bought another the same in a sale in the UK for 40 quid.
I saw exactly the same jacket in Oschner Sport here for CHF240
See above.
Although if you are Swiss, you definitely need the discounted 550 CHF pair although you need to appreciate that these will only take you so far as they will be in last season’s colours.
You didn’t actually say what you wanted the trousers for but it can make a big difference to what people should be recommending.
Zips are pretty essential for hiking over-trousers as you really need to take them on and off during the day - without removing your boots. They ought to be breathable and lightweight too.
A pair to walk the dog for half an hour our to use when pressure washing your drive don’t need zips, breathability or to be lightweight.
A pair to wear sailing the oceans need to be more like salopettes, high with braces to keep the water out but don’t need to be breathable.
etc…
Why?
I bow to the voice of experience:
No, I get that. Just wondered why dog walkers don’t need zips, lightweight fabric or breathability for the walks. My waterproofs aren’t posh or expensive (less than CHF 100) but they release with zips so I can get them off without having to remove my boots, and they have to be breathable/lightweight given the sweaty hilliness of our walks.
Pockets for the poo bag supply would be one thing I would have wished for, though.
I have a cheap pair and an expensive pair for cycling and golf. I don’t notice any difference between the two - both are not very breathable. Walking the dog is much more comfortable with an umbrella, only the wind may make pants unavoidable.