Leaky OMM Bottle replacement

I have the OMM bum bag (ultra 4L) which comes with a 500ml thin bottle to fit in the bottle holder. I like the bum bag and bottle shape but unfortunately the bottle doesn't seal well. The leak is not massive so you don't lose a lot of fluid it but it does mean it all gets rather sticky during a run if you put energy power in it. I think the leak is from the threads of the cap rather than the spout.

Anyone got any ideas where you can get a replacement bottle that doesn't leak and will fit in the OMM bum bag bottle holder?

Duncan