where to buy apps online

HI
I am another first time Android user with a Zen Touch 2.
Does anyone know how to get apps like Firefox, Documents to go, PowerAMP etc etc when the Zen Touch 2 isn't a phone. That is, I only have wifi which makes it is a problem getting access to the sites like Amazon, D3 etc app stores. I need to be able to purchase the app via the computer/internet and then load it to the Touch 2.

When it comes to the Android revolution I am enthusiastic, the ability to build a software (app) configuration in a portable device is way way overdue.
For a long time I have been annoyed by the fact that if I wanted to buy something with above average hardware specs I would be lumbered with a lot of soft/firmware I don't want and/or can

 

Thanks for your response you are very helpful.
Am I correct in my assumption the wifi purchase can be done by credit card, my searches revealed some reports of people trying to use paypal and not succeeding.
As for the multiple posts, I re-posted when the earlier one didn't appear, which apparently has occurred now. Thanks for the heads up.