Ricky,
I have the same rifle you do. It's an Ishapore 2A1 Enfield rifle that was imported by Navy Arms some years ago. They took some of the nicer rifles, bobbed the barrel, added the flash hider and "Jungle Carbine" wood to make .308 Carbines out of them.
The magazines are not interchangeable with the .303. The .308 mag has a flat, squared off bottom while the .303 British mag has an angled bottom.
Unless you've done it recently, have a gunsmith check the headspace on your Ishapore. Most will be okay. I would stick with .308 Nato marked ammo or moderately loaded .308 ammo (147gr) and avoid heavier loads. These rifles were built from the ground up to handle the .308 NATO (7.62x51) with the ~147gr bullet. They were not designed for heavy duty hunting loads.
These rifles work fine. The sights are typicially British - hard to see and use, but they will still perform well out at 200 yards if you line up the sights.
I think PROMAG had some aftermarket magazines available. If you find other manufacturers, please let me know too.