I just bought a new one at Bridge they have off road & adventure as well as road helmets & always helpful with me - but you also got SP motorcycles also on Marsh Barton and CMS at Pinhoe they have a fair few, plus goggles, oils etc etc. worth a trip to stock up on essentials
Plymouth you probably have to go to J&S accessories - loads there but you have to watch out for 'old stock' they are box shifters really - good deals but some stuff is years old - I went to buy a lid there a while back & looked at the date in one it was 7 years old still supposed new in box.
or GT ?