Looks like
this one. There are
plenty of places from which you could order it.
I'd order a derailleur online, if you can't easily get to a shop. Probably cheaper too. You're right about not trying to straighten it - chances are you'd never get indexing working smoothly again.
It's not so much the derailleur that is seven speed, as the gear-shift lever. That's where the indexing is set. You need a Shimano derailleur*, and you probably don't want the latest 9-11 speed, as you could run into problems with it being designed for narrow chains. However, something such as
this Acera should do the job.
As you probably know, you need the correct derailleur arm (cage) length, because the cage swings back and forth to take up slack in the chain, as you change between small and large sprockets and rings (which inherently require differing chain lengths). Looks as though you have a long-arm model, which is to be expected with your kind of bike. Short-arm ones will only cope with closer gears (and double rather than triple rings), as found on sports bikes.
If you fancy an upgrade, in your age of gear it's fine to mix and match, as long as you stick to Shimano, and you could go for a Deore or something. Rosebikes have
a good listing of what's compatible with what speeds.
Obviously you may as well change the cable while you're at it. If you haven't done an indexed gear before, it's not hard, but you're lining up the seven positions in the lever with the seven sprockets, so it's really a matter of cable adjustment. There are plenty of instructions on the Web. No doubt Park Tools will have some, for example.
* In the old days, you could use any derailleur with any shift lever and any sprockets. With indexing, the derailleur must move the right distance across the sprockets for each click. Different makes of derailleur move different distances for each mm of cable pull, different makes of sprocket cluster have different spacings, and different controls pull different lengths of cable per click. Bit of a minefield by comparison...
