Yes, it’s fine to buy a new helmet on the internet. What matters is that you have help selecting a helmet that fits you properly and is comfortable to wear. In a shop, the staff will be trained to measure your head and recommend a make and model to try. If you are buying from an internet site, be sure that you are allowed to try the helmet on and return it if it isn’t right for you. Wear the helmet at home for at least 10 minutes to decide if it is comfortable. Ask a friend to check that it is secure, not too tight and not too loose. Be wary of bargain priced helmets for sale on the internet. A really cheap helmet, in comparison to its normal retail price, is possibly a fake.