I have a lambskin 1/2 pad and use mine in top of my saddlecloth, i think equestrian clearance have a few different brands they have a cottage craft one for £25.99
Cottage Craft do a proper sheepskin one for about £40 which is nice quality. Most people (me included) tend to use them on top of a saddle cloth which when thinking about it rather defeats the object of the natural sheepskin! Do be aware that these pads are quite thick and will/can change the fit of your saddle.
Wear mine mainly over a saddlecloth. Both are stephens as the crappy cheap ones last a few months. I get my saddles fitted with the pads though, thats the way I like it done.
I have a Bartl which is lovelyquality. I put it under the saddlecloth which looks a little odd but improves the way my horse goes - she can be quite cold backed and it's actually decreased the liklihood of her bucking for the first few minutes i'm on her!
Surrey pet supplies website (http://www.surreypetsupplies.co.uk/)
have the half-pad for under £50, and they also do ones which are a full numnah/saddlecloth but with sheepskin only where the half-pad would sit for about £55. I'm saving up for one of those to make life more simple!
I have a rambo half sheepskin pad i bought from hope valley saddlery, mine was from their ebay shop for £25.
Its lovely, i wanted it as he needed a thicker pad under his back but not on his sides, it keeps his back warm without getting too hot, i love it!
I use a mattes one on george, on its own (they are supposed to provide close leg contact) although as others have said you should mention it to your saddler when getting fitted as they change the fit.
I borrowed one to try and used it on it's own but bottom edge of saddle rubbed his side - didn't seem to bother pony but stopped using it. It was too thick with a saddle cloth. Shame, it was a Nuumed, it was lovely.
Straight on his back as it soaks up the sweat and I am sure it's more comfortable on his back (so not with another numnah). Saddle fitter recommended it it's a Kieffer have washed it several times now and it's always come up looking like new paid about £35 but it was in a sale.
I have a Nuumed one that is half sheepskin and the rest a normal cotton numnah - so it is all-in-one. I just think its easier than using 2 thicknesses but maybe I'm missing the point of them?? My horse likes the sheepskin next to him as he can be a little cold-backed but not with this.
not quite the same but i have one of these, its a wool one and not full lined so doesnt change the shape of the saddle. i tried a sheepskin bartl one before but it was about an inch thick and must have made my saddle tight as she bucked with it on.