Primark! They have a great range, starting at £2 each. For a more formal pocket square, I'd invest in some plain white hankies - you can usually pick these up in Debenhams / M&S in a 3-pack for a tenner.
Anything a little more swankier, look at Reiss, Ted Baker, TM Lewin.. can't go wrong with the Primark ones tbf. I bought 5 for a tenner the other week.