We present parametrized broad-band spectral models valid at frequencies between 30 and 300 MHz for six bright radio sources selected from the 3C survey, spread in right ascension from 0 to 24 h. For each source, data from the literature are compiled and tied to a common flux density scale. These data are then used to parametrize an analytic polynomial spectral calibration model. The optimal polynomial order in each case is determined using the ratio of the Bayesian evidence for the candidate models. Maximum likelihood parameter values for each model are presented, with associated errors, and the percentage error in each model as a function of frequency is derived. These spectral models are intended as an initial reference for science from the new generation of low-frequency telescopes now coming online, with particular emphasis on the Low Frequency Array (LOFAR).