Your local asian shop will typically not stock sweet paprika. When you see 'Paprika' in American recipes, it refers to sweet paprika. The 'paprika' you get in asian shops is hot and spicy.
Sweet paprika is more common eastern European and Spanish cooking - so you might try some of new crop of Eastern European shops which have cropped up in most towns. Things like onion powder are harder to find - but eBay can be a good source for small quantities - as they typically don't fleece you on shipping costs. Oddly enough, I find very good quality sweet paprika at TK Maxx.
The shop Keith pointed to is also known as 'Just Ingredients'.
http://www.justingredients.co.uk/

