  Aims. An efficient means of locating calibrator sources for International
LOFAR is developed and used to determine the average density of usable
calibrator sources on the sky for subarcsecond observations at 140 MHz.
Methods. We used the multi-beaming capability of LOFAR to conduct a fast and
computationally inexpensive survey with the full International LOFAR array.
Sources were pre-selected on the basis of 325 MHz arcminute-scale flux density
using existing catalogues. By observing 30 different sources in each of the 12
sets of pointings per hour, we were able to inspect 630 sources in two hours to
determine if they possess a sufficiently bright compact component to be usable
as LOFAR delay calibrators. Results. Over 40% of the observed sources are
detected on multiple baselines between international stations and 86 are
classified as satisfactory calibrators. We show that a flat low-frequency
spectrum (from 74 to 325 MHz) is the best predictor of compactness at 140 MHz.
We extrapolate from our sample to show that the density of calibrators on the
sky that are sufficiently bright to calibrate dispersive and non-dispersive
delays for the International LOFAR using existing methods is 1.0 per square
degree. Conclusions. The observed density of satisfactory delay calibrator
sources means that observations with International LOFAR should be possible at
virtually any point in the sky, provided that a fast and efficient search using
the methodology described here is conducted prior to the observation to
identify the best calibrator.
