I use Sweet Squared to buy most of my nail art - lecente glitters, foils, pigment powders.
I had a set of nail art brushes from ebay, which were fine and still in use 5 years on.
For stamping I use Konad stamping polishes, konad & moyou plates
For nail art stickers, I use nailart.co.uk. Handy for the odd snowflake, snowman or reindeer when required!
EBay isn't necessarily a bad thing. There are a number of reputable traders that sell via eBay, you just need to keep an eye on who you are buying from.
There was a lady there once who was selling the best foils I've ever seen, sadly she had to retire due to ill health... I stockpiled enough for several lifetimes before she did because I don't expect I 'll ever find any as good as hers again.