To my knowledge, matey, a Mk4 Golf interior is a fit. I've seen leather seats taken from a Golf fitted to a Leon. So that I believe is simple swap.
 
Anything fits, if you have the ability to have subframes made or modified for you. Audi S3 and Golf Mk4 are the two that should, in theory, drop right in though.
 
i think most of the cars you've listed share the same platform so the seats (he fronts at least) should go straight in. i'd like the two front seats front an audi TT quattro sport!
 
r32 seats go in, but are rare and cost the earth
s3 recaro's "should" go in but im not sure how wide they are and if they are too wide for the leon
 
German eBay is the place to look for fancy seats. I've seen some stunning RS4 leather seats go for silly money on there.
 
You'd be surprised!

All I'll say, is keep well away from German eBay if you have a credit card, a desire for new seats and an inability to stop yourself buying fancy things for your car ;)
 
you could probably get a boat over there and back and it will still cost you less than this country
 
There is a definite bargain there for some lucky sole! Almost worth buying some standard R32 seats and offering a deal with them plus cash, just to get the leather ones!
 
i've got black mk4 golf v5 seats in my leon. fitted right in, had to change the air bag connectors over though but that was a piece of pi55. look a good sight better, they're sort of a simmilar style to the recaros, a lot better / more supportive than the standard seats i had in mine.

i've also heard that octavia seats will fit straight in but i'm not 100%