  Small single crystals of Rb$_3$Ni$_2$(NO$_3$)$_7$ were obtained by
crystallization from anhydrous nitric acid solution of rubidium nitrate and
nickel nitrate hexahydrate. The basic elements of the crystal structure of this
new compound are isolated spin-1 two-leg ladders of Ni$^{2+}$-ions connected by
(NO$_3$)$^-$ groups. The experimental data show the absence of long range
magnetic order at T $\geq 2$~K. LDA+U calculations and the detailed analysis of
the experimental data, i.e. of the magnetic susceptibility, the specific heat
in magnetic fields up to 9~T, the magnetization, and of the high-frequency
electron spin resonance data, enable quantitative estimates of the relevant
parameters of the $S=1$ ladders in Rb$_3$Ni$_2$(NO$_3$)$_7$ . The rung-coupling
$J_1 = 10.5$~K, the leg-coupling $J_2=1.6$~K, and the uniaxial anisotropy $|A|
= 179$~GHz are obtained. The scenario of spin liquid quantum ground state is
further corroborated by quantum Monte Carlo simulations of the magnetic
susceptibility.
