If you try Sports Direct, then go to one of their local stores for a comparison of your needs. Then have a look at their on-line offers, for the same product. I have daily email promotions and offers from them, and some great bargains can be obtained, especially if they are overstocks or store clearance returns. Delivery is usually next working day, which you can usually track.
If you're wearing trainers for their intended purpose - ie sport - then you really need to make sure they're comfortable.
I buy mine for Sports Direct, where I can try them on and walk around in them.
Then whilst in the shop I check if they're cheaper online using Shop Savvy app on my phone. If they're only a couple of quid cheaper online I buy them from the shop, otherwise they go back on the shelf and I buy online
As someone who claims to still be a reasonably competent athlete, I would never go anywhere except a proper running shop. I am happy to pay a bit extra for expert advice and from people who understand my needs.
And when the shoes are worn out, I use them for gardening.
I have to trawl through the Ebay pages when in need of new trainers, as I have "canal barges" for feet. Its not always because of the size but more recently the style of the trainer. I have used others but the last few pairs were from Ebay, brand new, and more often than not with a 30% saving, and you know me I like the word "saving", lol!
Just for info, my last pair were from a shop in Cardiff, but can't remember their name, sorry.