Asda is 99.7% exactly the same as Wal-mart. There's 3 differences. The name, the primary color (wal-mart = blue asda = green) and the currency sign.
The merchandise is exactly the same stuff, and i mean exactly. They have a George clothing section both in the US and the UK for example.
And if you ever happen to be in the US during September/October, go to wal-mart, look at the halloween stuff. Then go to asda. Same stock, same packaging, same name.
It's everything you can buy in the US and it's relatively cheap. I probably get around 60-70% of my entire seasons haul from there.
They also sell alot of imports from Party City (another US seasonal chain)
Tesco have started to show a big boost in the amount of floorspace they devote to halloween in the past couple years. Last year having being 2 huge sections and the 4 isles surrounding them.
Then i'd say ebay is your best bet. Followed by pound stores, party stores and card stores. The last two being the most expensive.
Just thought i'd try and help add a little info but yeah there was a UK merch thread last year and i'm sure this year it'll be just as popular.