Help with new cushions on sofa?

[Fluffyhood]

I don't know the correct names for describing sofa types so please bear with me as I try to explain!
I've had my sofa for 6-7 years maybe and there is nothing really wrong with it except for the fact I haven't turned/ plumped the back cushions regularly so they are starting to look old and lose their shape.
They are 'normal' sofa cushions that are fitted to the couch so I thought I could get rid of them and buy loose cushions to use instead. The actual back of the sofa is shorter than the cushions so using loose cushions would look fine.
Has anybody got any recommendations on where to buy loose cushions big and sturdy enough to use for a couch? (I'd looked in various shops over the past few months but not found what I wanted)
And has anybody else done the same and is happy with the outcome? Really trying to avoid buying a new sofa as the rest of it is in great condition apart from the cushions
Thanks!

Could you get the cushion pads replaced instead?

I agree with choppolata, see if someone could restuff the back cushions.

Otherwise, you do get 50x50 and 60x60 cushions, try Next or John Lewis, but I think they will look odd without a back. Could you stand the large cushions in front of the saggy back cushions to disguise them, together with maybe a couple of throws folded over the back?

What about something from somewhere like this
www.efoam.co.uk/?gclid=EAIaIQobChMIgZe3hJfg7gIVi57tCh0GIAEIEAAYASAAEgK28PD_BwE

I had the same issue with mine. I got five 50x50cm cushions from M&S and a nice NEXT throw blanket to go over the arm and it looks much better.

How about getting some cut to size cushion foam would that work?