In the USA I have had good luck buying my trailer tires from:
www.tirerack.com

They ship same or next day and you get them installed locally for
cheap. You get exactly what you want every time. I think I paid $58
each for Goodyear Marathon trailer tires last time. Not more than I
would expect to pay for auto/LT tires if bought from a shop.

I always change my trailer tires after 3 years of use no matter how
they look. It's not worth the risk of a blowout to take them much
beyond that.

Pat Russette
CQ