LeMieux now do one with pockets as well called 'pro-sorb'- I've just got mine. I also have mattes half pad corrections.
On the mattes, the pockets are on the top - so you have sheepskin, a layer of fabric, then a layer of quilt - the shims go between the fabric layer and the quilt with the quilt opening on the top. I love the mattes: the sheepskin is lovely and thick. I haven't had a 'new' mattes as tend to get mine in v good second hand condition as cheaper, but mine feel more like a thick fleece feel in terms of texture (although it is sheepskin, not fleece).
My lemieux pro-sorb is a thinner, silkier type of sheepskin - lovely and will do the job beautifully, but perhaps not quite so protective on an extra extra sensitive horse. This also has a spine free channel. The pockets are different on these. Whilst the mattes do a 2 pocket and 4 pocket correction, lemieux started by only doing a 4 pocket. They have changed and now only do 2 pocket (a long pocket the whole length of the pad). The reason behind this is that it avoids there being an empty non-pressure bearing area in the middle where the seam would go. I worried about my front shims sliding backwards without a seam, as I shim the front more than the back. Having had an hours lesson using it today, I had no problems and the shims stay put. In this pad, the pockets open up from the underneath instead - you kind of lift the sheepskin away from the quilted layer.
My only quibble with the lemieux is that their shims are very thick - although a great material as they don't flatten over time. I found that with my gulleted treeless today I had too much padding in and as a result I couldn't lift the spine of the pad into the gullet of the saddle. However I do have a lot in there so can remove a layer and see how that feels. If you need to shim for asymmestry or wastage, you'll probably need some thin felt shims aswell as the ones that come with this pad as its surprising how little you need to make a huge difference be felt...
What treeless are you using it with? No Angel has a non-sheepskin shim pad that she's selling I think - might be woth contacting her?
The other thing is you can often pick up cheapish good quality half pads on ebay or wherever second hand but in really good condition, without shims - if you're handy with a needle, you can always stitch on some pockets of your own...
The thing with mattes (and hopefully lemieux - ask me in a yr or so for sure lol), is that if they are looked after, they will stay good as new really; brush it after each ride quickely to stop sweat settling on the surface and causing it to matte and it stays lovely...