Trooper side vents

I am currently tidying up my '92 Trooper in preparation for the new season as the old bus was looking a bit tatty. I would now like to replace both the side (fridge?) vents but can't seem to find any in the correct size, upper 478mm X 183, lower 472mm X 170. Can anyone point me in the right direction? I would prefer good serviceable second hand items but beggars can't be choosers 

Definitely still available and the sizes are quite common. Dometic LS100 is the upper and the lower is often an LS200. They often turn up on eBay as pairs. I seem to remember the upper bonds to the fridge flue outlet and the lower pairs with the hob. Leisure Shop Direct sell them but they are quite pricey new.

Sorry for the slow response Tattybogle but thanks for your reply 
I'd seen the LS100 and 200 vents you mention on eBay but the sizes never quite matched my carefully taken measurements. They are obviously an approximation (I hope!). The new ones are just within my budget so I'll be ordering imminently. 
Thanks again