EmmaEvs,
This is the helmet I bought.
http://www.salomon.com/uk/#/ski/helmets/helmets/kiana
It is meant for 'young girls' but I have a small head and it fitted me perfectly. I tried on about 5 different helmets and this one was really comfy and stayed in place when I waggled my head even without the strap on. I found once I'd put my helmet on I just totally forgot about it, it was so comfy and light, and I could ski all day with no problems.
As you can see, it has holes in, for vents. It also has an attachment at the back for fixing your goggles to, which is really handy because it means you can put them up on top of your helmet when you're ski-ing and you don't need them, and you can't lose them because they can't fall right off.
It also has ear muffs, which kept my ears nice and warm, although unfortunately I couldn't fit my mobile phone under them so I had to take my helmet off to use my phone.
It only cost £40, and the man in the ski shop said it would be fine for me, even though I am an adult.
If you're planning on buying a helmet, I think you need to actually try them on so don't try buying one over the internet. Also you need to bring your goggles with you to make sure the helmet fits with the goggles.
I was going to rent a helmet, like you did, but I particularly wanted my daughter to wear one, and I knew she was going to want some trendy new one and would refuse to wear an unstylish rental one, so we both went shopping for helmets.
Ally