Cheers all. I do already have new valves, tape and sealant so it'll just be one insert, I'll try and get some closed-cell foam first and make my own.
I did have a stretched, rattly insert inside my other wheel and it was killing me to know it was just banging about sounding like crap, so I had to take it straight out and cut it down. So I'd rather avoid one that's prone to stretching.