Hello,
The starter sets as others have observed have mostly sold out. If you can find independent gamng stores in your area, it might be worth giving them a call as they might have one or two lurking at the back of shelves.
Noble Knight and Agents of Gaming seem to be the only 2 places left selling new models, otherwise you will have to resort to eBay. The common ships for most races are generally not too hard to find. What you may struggle to track down are the last wave of ships that Mongoose produced as part of the "Armageddon" expansion. These include very large ships for each race that were set after the main run of the show ended (imagine Centauri, Narn etc equivelents to the Victory and Excalibur and you get the idea). These ships were only in production for a short amount of time and not many were made. The exception seems to be the Centauri as their Adira and Liati advanced ships are still available fom Noble Knight.
In terms of races, you have all the main races present in the show.
Earth Alliance (3 variants representing the different eras available)
Minbari
Centauri
Narn
Interstellar Alliance
Vorlons
Shadows
PsiCorps
Dilgar
Drakh
Vree
Gaim
Drazi
Abbai
Brakiri
The races in italics can be fielded independently or together as part of a "League of non-aligned worlds" fleet.
The balance between the races in 2nd edition is improved although some are slightly stronger than others. Do you have any idea what fleet you are looking to field?
Some races are easier to buy than others as they have been in production for longer. PsiCorps, Gaim, Drakh and Dilgar fleets will be tricky as you will probably have to scour eBay for the ships you need. Other races will be easier to find.