I’m looking for some parts but can’t begin to properly describe them to google them. 
 

On the parcel shelf of my mk 2.5 focus, there are two sheaths which cover shards of metal which hold the shelf in place. Both have fallen out over time and now the shelf frequently comes off. They are maybe 5cm long, cylindrical and rubber. 
 

Any idea how I can find replacements?

Have a look on catcar or zap, which have parts list and exploded views which should enable you to identify the part and finis code.

https://www.catcar.info/ford/?lang=en

https://ford.7zap.com/en/car/

Not available separately from Ford I'm afraid.

I think you could probably 'make' a pair by cutting a rubber pipe down to size.  Just need to measure the metal rods to work out the internal diameter of the pipe.

I don't know quite what is required, but would some heat shrink tubing suffice ?

I’d expect heat shrink wouldn’t be thick enough but some fuel pipe or similar would probably do the job

Worst case or as a temp fix - electrical tape 👍