You should have no problems filling your wallet in Tallinn - cash machines (ATMs) are never hard to find. Big banks are usually your best bet for exchanging currency. At night and on weekends, however, decent exchanges are more elusive. Tavid at Aia 5 has good rates and also runs an all-night exchange window that will do in a pinch. Their night rates aren’t as good as what they are during the daytime. Beware of bright blue and yellow Monex exchanges around town. These have convenient locations and working hours, but generally give unfavourable rates and even  charge a €3.20 - 6.40 commission on each transaction.