yeah i know what you mean - I've been reading all sorts over the past few weeks and didn't feel much wiser! Haha. I made some of my decorations myself, the little roses and such, as I had some fimo moulds to begin with. Most things I have bought have been from two sites mainly.
I go to www.etsy.com as they have a wide range of shops who sell different things to get you going. A general 'decoden' search will bring up a whole range and you can find loads of great deals on there.
The second site I have used is sophieandtoffee.com. It is a site that is based in singapore and they have a great range of decoden items.
I have just recieved an order from them which I am extremely happy with - everything came in great condition and great quality.
That order cost around £6 I think including shipping. It's not the cheapest place to buy from but their stock is awesome haha.
Whipped-cream-wise I have used 2 different ones so far. The first was air dry clay called fuwa-fuwa
http://sophieandtoffee.com/blog/wp-content/uploads/2009/12/DSC03662-copy.jpg
I found this easy to use but for me, it didn't dry well enough and wasn't very durable. But bear in mind that I used this on a phone case so it was quite thickly applied. A lot of people use it for rings and such and say it works well.
The second was 'Grace' resin clay. It too is air-dry and it worked fantastic! It does take around 3 days to dry fully once applied though - so be prepared to check every few days and try not to drop anything on it when its drying... (experience). I bought this version:
http://sophieandtoffee.com/item.php?item_id=285&category_id=31
whoch as you can see comes in a plastic tube. You then put it into a piping bag and apply a tip and off you go! That particular version you do have to warm first - I personally used a hairdryer. None of these options are cheap enough for me as I'm looking to sell these on! My next try is going to be decorators caulk which cost me around £3 for 300ml. I'll let you know how that one goes!
A quick note as well - if you do use a piping bag - either double bag if you are using a thin plastic one or use one that is meant for cakes so its a bit thicker - the clays can get quite thick to pipe on and that can mean your bag can burst...(again...experience! I lost half my bag on my hands!!!)
By the way - this is the only thing I have really made so far - it's the case for my iPhone 3gs.
This was made from the Grace clay - and I know its a bit messy but when the bag burst - I could no longer make it all smoothl whipped looking so I had to go for icing look :p haha. I bought the large rose and pendant base both off etsy along with the rhinestones and pearls. Everything else I made. I think this proves more than anything that the case can really have whatever you like on it - mine just HAD to have Professor Layton on it so I made him from hama beads.
Now I think that's it for now, but any more questions or attemps - lemmie know!