Paint

Vallejo makes some sets of paint for things like modern USAF, modern & WWII USN & USMC, Russia/Warsaw pact, etc.  You get a half dozen colors, but still may need to supplement with common colors (black, white, steel, etc).  Get the sets you need & build your stock over time.  

Vallejo is a good paint, but these may be in their Model Air line of colors which are optimized for airbrush.  They may be a bit thin to hand brush, but it is possible.

Vallejo Model Color is their product optimized for handbrushing - it just doesn't come in the sets.  

Don't overlook seemingly miniature paint brands, Citadel, Games Workshop, Reaper, Army Painter.  Heck, look at the craft paint aisle in the local Hobby Lobby too.  Any will work with appropirate technique

Times are changing.  On of the popular vendors has greatly reduced the number of colors in its line (leading to rumors that vendor has gone).

As far as where to buy, many craft stores (like Hobby Lobby, Michaels, etc) carry model paints, but each carries only a few vendors.  Walmart carries acrylic paints.

Then, you need to decide on what type of paint- acrylic, lacquer, enamel, or a variety of hybrid proprietary mixes.  Check below in the Painting and Airbrushing forum for discussions on advantages of various types.

Also, you can find auto paints at auto parts stores.  Many car modelers like auto paints.

Do you have an airbrush or will you be brush painting.

Do you prefer acrylics,lacquers,enamels,or acrylic/lacquers like Tamiya or AK-Real

Yes some manufacturers like Vallejo and AK do make sets for various projects.

What are you building.

Some good sites for paint are  Spraygunner,Scalehobbyist,Squadron,and Spruebrothers.

That's the way we do it,when starting a project we decide what colors we need,do I have some from previous projects,who makes them,are they easily available,do I like them.

First couple of times it does seem like your buying a lot of paint,but after some time you will acummulate common colors.

but you need to answer the first two questions before you proceed