Well I went to air up this morning and the valve stem just about came off in my hand, so it's finished. My question is, I thought I had seen some people replacing theirs with some other heavy duty or after market tube. Where do I look or do I just get im touch with SM customer service?

You can just go and get your standard 12" bicycle tire, it should fit in there fine. If you want to make sure you can take the old tube out and bring it with you to the store. You can contact Sun Mountain and they might get you one for free, but it will take a little more time and they are only a buck or two anyways.

I've ordered replacement inner tubes from Sun Mountain's website a year ago.

It's a knuckle buster for sure. I took it to the local golf shop and had them do it. Had all 3 replaced for $20.