Does anyone know where I can get the plastic retainer for the engine cover support arm?

Have you considered a gas strut kit?
Far easier to use than that annoying prop.
Several available such as 
https://twosrus.com/catalog/product_info.php?cPath=22_30&products_id=133
http://www.ebay.com/itm/200981736560
 
Anyway here is one source for that plastic clip.
http://twosrus.com/catalog/product_info.php?products_id=809
 

I think admin used one from some model celica at one stage and from memory it could have moved on to Rick Plane
 

Hello,
same issue with my AW11. Is the 2RUS MKII clip linked above compatible, or is there another part no?
SteveB

Reddtarga
 
Anyway here is one source for that plastic clip.
http://twosrus.com/catalog/product_info.php?products_id=809
 

I got one of these from Lithia.  It's great.  My problem is the other end also falls out.  Any ideas how to secure "the base"?

Www.primemr2.com
Under exterior parts
$3 usd plus postage

Go the engine lid gas strut guys. It's more expensive then a plastic clip but once you have one you will wonder why it wasn't fitted as standard equipment. Available for SW20 and AW11.

You can buy the plastic clamp from your local Toyota. I can't remember the cost, but it was only a couple of dollars. Buy a spare with the money you save on postage.
 
53455-17040 Clamp, Hood Support
 
Edit: Woops, didn't realise the thread was 4 weeks old but still relevant information for anyone.

Thanks Lumix and others.
I didn't realise both ends of the prop had plastic clips prone to breaking. I need the lower swivel one.
SB