Builders merchants. Usually cheaper than the DIY places.
Have to say I'm very fond of our Trailerbarrow which was a tad more expensive - you pull rather than push it, and it's light weight enough (plastic) for kids to deal with, very stable too. Inherited from my father when he moved to a smaller garden plot about 5 years ago, must be easily 10-12 years old used most days too.
if its your tyre that has gone . just buy a new tyre i bought one in B&Q a couple of days ago "greentyre"( google it ) it will never burst or stop working , and only cost £17 , cheaper than a new wheelbarrow./
i got a new barrow from Arco was £20 but did use company discount. But gibsons at Darwen do a replacement wheel for £12 and they will post out, I got one posted.
hope this helps