My neighbour has a 1995 something or other ATB with Acera X 7 speed triple drive trains, she’s destroyed the rear mech and to be fair the rings and cassette look screwed, so no doubt the chain is – I suspect its original!

Where is the best place to get 7 speed replacements that fit?

My first port of call would prob be SJS…

https://www.sjscycles.co.uk/cassettes-7-speed/

8 and most 9 speed stuff will work (except the cassette, of course). If you want 7 speed my first ports of call probably would be SJS Cycles or eBay.

LOL, clearly I took too long to type that 🙂

Check it is a cassette and not a freewheel before buying anything.   SJS either way though.

Rosebikes and Bike24 have a few 7 speed bits.

Local bike shop?

8 and most 9 speed stuff will work

Ah, remember the days when (nearly) everything was compatible with (nearly) everything else? Good old days they were, and all of this was just fields…

If the cassette, chain, rear mech and rings are shot, you’re not really limited to 7 speed anymore. You might find a deal on some 9 speed stuff. After all, you’re only shifter away.

I realise as I say that, there’s a good chance the shifters are integrated to the brakes

Amazon is also great if you know what you need

8 and most 9 speed stuff will work (except the cassette, of course). If you want 7 speed my first ports of call probably would be SJS Cycles or eBay.

If the cassette, chain, rear mech and rings are shot, you’re not really limited to 7 speed anymore. You might find a deal on some 9 speed stuff. After all, you’re only shifter away.

If it’s a freehub and is spaced for 7 speed you won’t fit an 8 speed on however a 9 will fit with a sprocket removed (did this for my sister in law), I found a 12t end sprocket to replace the 12t and 11t end so it doesn’t look wrong.

Decathlon also do Microshift kit if you’re working to a tight budget.