I'd recommend looking at either Lynx or Communica gear. Both are designed specifically for microlights: which are both open cockpit and hellishly noisy at times.
I've used both, and whilst I own Communica (having got a good second hand deal a few years back), would for preference go for
Lynx which I think is a bit more robust. The headsets work well in an open cockpit, and both companies do helmets to go with their headsets.
Most GA equipment is designed for quieter, draft-free cockpits, and tends not to be as good for an open cockpit aeroplane as headsets and helmets designed specifically for microlights.
If buying a helmet, make sure you have a visor latch, otherwise in a lot of cockpits the visors can occasionally decide to ride up at irritating moments.
If you want to browse, then your best bet is to go to Popham on the May bank holiday weekend when the big microlight fly-in is on.
G