Where is the best place to buy some.
You could probably find a stall at your local market selling earphones for about £2-£3! They would be useless for music, but sound quality doesn't really matter for this application. (If you actually
do want to listen to music on them, then be prepared to pay 10-50 times as much!)
I suggest that you pick the type that go
IN your ears not
ON your ears, and then (as suggested above) only wear one side to keep the other ear free for hearing psycho drivers coming at you from behind!
I don't imagine that your market stall holder will want you inserting earphones in your ears before buying them to make sure that they fit! I have had some that keep falling out. Look for ones that have 3 or 4 different sized rubber sleeves with them - you just pick the size of sleeve that gives a good fit.