I'm in need of an indoor omni antenna (actually two) in the 550-600mHz range. I've found a few with 5-9dBi gain from Italian manufacturers. And while we are well versed in importing, we find their pricing....well, shockingly high. Any US ideas?
In my experience, when people want to refer to an isotropic or "0 dBi" antenna (which is a purely theoretical reference - no perfectly-isotropic antenna can actually exist) they use the word "isotropic".
In common usage, the term "omni" for an antenna usually refers an antenna such as a monopole or dipole, which has a uniform radiation pattern in a two-axis circular pattern, and is directional in only one axis. I assume that's what the original poster is looking for.
Antenna gain is not always quoted w/r/t isotropic. Some manufacturers quote gain that way (as dBi). Others quote it w/r/t a half-wave dipole (as dBd).
Antenna vendors which quote antenna gain in "dB" (without telling you what they're using as the 0 dB reference) are generally using dBi numbers, because they're bigger by 2.15 and thus look better in the ads :-)
Now, to the original request: as to needing antennas in the 550-600 MHz range (which is what I assume you're looking for since you said "indoor")... that's still part of the US UHF-TV band, I think, and so not one for which small narrow-band antennas would have much of a commodity market, hence not many cheap commercial antennas available (or so I suspect). Most coverage here is probably broad-band (often untuned) receive-only antennas.
How much gain do you need? Receive-only, or is transmission required? Can you satisfy your needs with something as simple as a wire whip of the correct length, soldered to a BNC or similar connector, maybe with a counterpoise wire attached to the ground shell, or a couple of ground radials sticking out sidewise? Does it need to be pretty, or child-safe, or capable of surviving the landing and attack of a hyacinthine macaw? 50 ohm, 75 ohm, or something more exotic?
An antenna is termed omni-directional if it possesses a circular radiation pattern in the azimuthal plane. The same pattern in the elevation plane can be doughnut-shaped or something else. An isotropic antenna is a theoretical construct with a spherical pattern in 3-space. As its pattern is spherical, it's useful as a basis for comparison of antenna gain (azimuth & elevation). Gain expressed in "dBi" designates the units for this comparison. An isotropic antenna can be approached in practice using 3 orthogonal electrically-small (in terms of a wavelength) dipoles or loops (often called "magnetic" loops by ham radio operators). Sincerely,