The LPE growth technique was used to prepare the thick films of Bi-substituted iron garnets for optical isolators. Phase diagrams of the complex flux systems were investigated. It was shown that in systems with rare-earth oxides concentration less than 0,5 mol. % stabilization of the growth rate takes place. According to our investigations Y, Ho and Dy have more "equilibrium" cation radii for dodechahedral sublattice of garnet structure. That is why we selected (lin,B1)3(Fe,Ga)5O12 garnet film compositions with Ln = Y, Ho and Dy for the preparation of more perfect layers. The duration of LPE processes was in the range 2,5 to 12 hours. Bi content in the prepared films was 1,5-2,0 atoms per formula unit. The uncracked samples with 0,2-0,4 mm sum thickness observed the Faraday rotation of more than 45° at the wavelength range 800-1300 mm. The absorption spectra for high level of Bi substitution and different Ln were also investigated.