There are many online stores from which you can buy almost any diabolo in existence:
www.juggleart.com.au - A great, cheapish store for people around Australia looking to buy items
individually.
www.firetoys.co.uk - A store in the UK with very fair prices and a huge range of diabolos.
www.diabolomoves.co.uk - A store in the UK run by an actual member of the forums. The very
BEST place to buy Sundia and Taibolo brand equipment, diabolos,
handsticks, string, and all the accesories.
All of these sites give a brief, or in some cases, quite in-depth summary of each diabolo, and may help you decide which one to begin with.
In my opinion, the Sundia Fly diabolo with a bearing axle is the way to go for beginners. The bearing axle makes keeping spin while you learn your first steps all that much easier. They are also very nice all-round diabolos even when you become a more advanced diaboloist. The bearing axle can also be fixed, so that the Fly will operate like any other diabolo.
The Fly also has wide and narrow axle variants, I'd recommend the wide axled one because of it's increased flexibility. (Ease of grinding tricks and the like aren't restricted.)
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Of course, there are other options to choose from. Many people would recommend a Harlequin, a smaller diabolo made by 'Mr. Babache'. This diabolo also comes in two variants, a larger and smaller version. Because of the diabolo's durability, it will easily stand up to the abuse of a learning diaboloist.
Between the two variants, I'd recommend the smaller one for a beginner. Smaller diabolos are usually easier to control and get to grips with for new people. However, if you're already quite comfortable with the basics of using diabolos, the larger version will have a longer 'Play-Life' before you feel that you need to replace it.
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The Finesse, Circus and Sun diabolos, made by Mr. Babache, Henry's and Sundia respectively, are the fixed-axle diabolos that I would recommend above all else, though, after Sundia Flys.
All three of these diabolos are very easy to customize to your liking, and are extremely durable. They are of professional quality and you may never be drawn to buy a 'replacement' for them at all.
The three of them are pretty much the 'Jacks of all trades' as far as diabolos go, and there are no purposes that these diabolos are incapable of serving.
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That's all I've got time to write, sorry.
If you've got an interest in any of these diabolos, either look one up on one of those stores (Above) or search them into the forums here for lots more information.