Bonsai is a pruning and shaping style, not a particular plant. When you choose a bonsai already grown, you look for a shape and size that appeals to you and then you work to maintain the shape and size. Bonsai is a very fluid art form as live plants are not stagnant (they are constantly growing and changing). Pruning is a constant to keep the shape but repotting and root pruning are done every 3 or 4 or 5 years, depending upon the plant and its health.
The problem with both calliopemcgraw's and Brandon's plants is they are junipers. Rampant growers that don't like inside the house conditions. The reason the trees are losing leaves is they are too warm. Find cool but sunny locations. Water when the soil starts to dry. Warning: In the summertime, that might be 3 times a day.
Bonsai purchased from the store that don't cost an arm and a leg are what I call "instant bonsai" - they have been mass produced and don't have all the attributes real bonsai have - it's a losing battle but not a lost cause. You will both have to do some research to learn the proper care for your new plants.
All my bonsai were started from seed or small plants and have been trained for years to achieve the desired look. I don't grow small bonsai because I don't have the patience for the constant care they need - the smaller the pot, the harder to maintain. New bonsai will stay in 'training' pots for at least 5 years before I even think about putting them into bonsai pots - that's kind of the end of the project, not the beginning.
If those junipers were mine, I would back up and repot them into larger pots (1 gallon) for a few years while I trained and maintained and learned what I was doing. But, junipers are not houseplants. If you really want to get into bonsai and keep them inside, find a Ficus, caudiciform begonia, Money Tree or Schefflera. The slower growing the plant, the easier it is to bonsai.
