Can you wait and buy it when you arrive? NO
Money belts are designed to hide valuables, specifically currency notes, credit cards, emergency contact details and documents. The idea is to keep the goodies somewhere that makes it hard for pick pockets, robbers and opportunist thieves to find them.
This has good and bad points. On the positive side, any thief who isn't wise to the plan will go away,after emptying your pockets and bags, convinced you don't have anything. On the negative side, a thief who is aware of the plan and has you cornered may well insist on you opening your shirt, dropping your trousers / skirt to see if you are wearing a money pouch somewhere well hidden.
Overall, the good usually out weighs the bad as determined robbers with a big knife are much rarer than pickpockets and opportunists.
There are two popular styles of money belt; flesh coloured cotton or canvas pouches on straps, and trouser belts with a zipped pocket running round the inside of them.
Pouches are attached by straps, usually around your tummy or chest, but may also be around the waist, the top of a leg or hung from a shoulder. They have the advantage of being hidden under clothing so pickpockets will find it impossible to reach them, and all your goodies are out of sight. The disadvantages are three fold. Firstly they can become sweaty in hot climates, secondly you need to partly undress to get at them and thirdly, if they do get seen by a thief, their purpose cannot be disguised.
Belts are worn as normal trouser belts, and thus are perhaps less handy for women in skirts or dresses. Advantages include being an obvious item of clothing and having nothing secretive about them, and being on the outside of clothing, so they wont become sweaty and cause rashes and itching. It is also easier to unobtrusively open a belt compartment than access a pouch fastened to your chest or thigh. Disadvantages are the already mentioned lack of compatibility with dresses and skirts, and the possibility that a determined robber may decide to remove your belt as being worth a few shillings anyway.
Generally of course, if you are unfortunate enough to meet anyone with the power to remove items of your clothing, its probably safer to hand over everything you own before they discover that you are holding things back.
Regardless of the system you choose, a money belt is an excellent item to have. It means you can leave rucksacks in rooms without carrying all your important documents in your pockets, you can keep cash and docs safe whilst you sleep, and when your rucksack goes on the roof of a taxi or the boot of a coach, your key documents stay with you.