I'm uncertain if you're in the UK or elsewhere, because of your mention of dollars. I'm also not familiar with the websites you mention. Either way, take the option to "browse catalogue" on the left side of the screen on the Fisher Scientific website:
http://www.fisher.co.uk/
Type "caesium" into the "Keywords/Chemical Name/ Structure" box and click "search". Work through the list until you reach caesium as the element. You should be able to convert to dollars easily if you need too. Move on to look for sodium etc.
This website will give you an idea of the current market price without being ripped off. However, they won't supply you with anything unless you have bona-fide credentials detailing the purpose to which their products will be used and the nature of your work. They will not supply private individuals.
As to fun experiments with "alkali metal derivatives", I seriously advise you to forget it. You're putting yourself in great danger.