It all depends on what you want – and how much you want to pay.
If you want vintage ties, you missed the great Modculture sell-off. However, eBay should be a good source of vintage items, as will your local charity shop if you’re prepared to do the leg work.
Want new? Just wander round your high street in the first instance, slim is ‘in’ right now, so you might get lucky at any number of retailers. Want something a little more ‘Mod’? Check out Peckham Rye for both both slim and knitted ties, DNA Groove has some interesting styles and Jump The Gun has some decent knitted numbers too. In short, there are plenty out there.