Woolies used to do some good tinsel, but maybe it is too late for that now!Places like Hallmark or whatever branch of card shop you have should also do it .. although it may be a bit costly.
Have you looked in charity shops _ Could be worth a try.

Not the high street, but Homebase are good, also check out your local garden centres. Sounds odd but most of our local ones have fabulous decos...

BHS had some nice tinsel, lots of different colours.

BHS always used to do solve my tinsel needs. Robust. Lasts for two or three years.

I'd recommend DIY stores like B and Q, homebase or wickes as they have specialist sections for Christmas decorations. As they specialise in garden decor they should have some quality tinsel (as it needs to be weather resistant) which is affordable.

http://www.homebase.co.uk/webapp/wcs/stores/servlet/Browse?storeId=20001&lan...

Asda I brought some lovely fluffy tinsel fromthere and it was thick as well and reasonably priced.