Obviously normal kantors are a ripoff and exchanging money in the bank is downright stupid.
Normal kantors are most certainly not a rip-off (although kantors at airports and five-star usually are).
My kantor of choice is offering:
USA 321,50 323,50 100 USD
EURO 412 414 100 EUR
W.BRYTANIA 511 515 100 GBP
SZWAJCARIA 343 346 100 CHF
Conti
Al. Jana Pawa II 46/48
conti.waw.pl/cennik.html