I have kept Bonsai for years and they are very simple to look after. Most people fail because they forget to water them - that's all. Bonsai have very small pots so they dry out quickly.
I have found too much fertiliser has a negative effect, leaves drop off etc. I fertilise once a week and water daily. No other secrets for me. In spring/summer I prune where necessary. My current tree is about 12 years old and I've had it since it was 3 or 4 years old.
I've got one that is 18years old that I my sister and I bought for my old sister, who almost killed it within a few weeks, so my mum stole it off her and I now look after it.
It is looking a bit wild and woolly as I haven't trimmed it yet. I left it all winter and am going to do it again, but am just deciding whether to repot it.
I never fertilise mine, so would love to know what you use, I'm sure mine will love me for that!
I water whenever the soil starts to look dry, so in the summer that is everyday, but in the winter it is every few days, sometimes a week, depending on how cold it gets in the house.
you can get them in Wyvales etc but i bet it's like anything, if you go to a fish breeder you generally get better fish and so on so i suppose your local Bonsai club would be the best place to go although it might be a bit more expensive?
I'm not an expert keeper but I've had one large one for about 2 years and a smaller one for about 1 year. as mentioned by others you only water when the soil looks dry, a really small thin bit of wood/plastic to poke down into the soil will tell you how wet/dry the soil is underneath the surface but don't poke about too much because you may damage the roots.
I use a test-tube-style drip feeder especially for bonzais as a fertliser. They seem to work better for me than adding the liquid ferts to the watering can once a week.
On where to get one - it seems that every garden centre has a section for them. My 2 year old I bought at 1/2 price from a DIY shop.