I too have 'bonsai' and am experienced in their care but have trouble keeping shop bought ones alive despite changing their soil and doing everything else that one should.
I believe they are dying before they are sold mainly due to no/poor care at the retailers although I did rescue a Serissa from a bin once and turned it into an exposed root healthy tree.
As long as you accept that the lifespan is going to be short (unless you are lucky) and don't pay the earth then choose one with a good trunk, damp soil and no dead branches and you may start a lifelong interest.
Should you happen to live within reach of Lingfield a visit to Peter Chan's 'Heron's nursery' is well worth it.
http://www.herons.co.uk/