SPS Varmints are all short action, there are no long actions available in that line. The regular SPS line can be had in both SA and LA. The only Remington long action .308 that you will find is on the M24.
For quality aftermarket magazine kits you will pay a bit but the Badger Ordnance M5 has been around for a while and is top quality. Also VBull is reported to be very good as well. Then of course you could always upgrad the stock on your SPS Varmint to an AICS chassis and run their detachable mag system. They are all quite a bit more expensive than the Cabela's system, but also a heck of a lot more sturdy as well. The Badger will require inletting of the stock as well.