I usually design my own templates for Joomla, from scratch using HTML, CSS and Photoshop. But you indicated that you wanted to download templates, so here you go:

1. To find templates, my favorite sites are:
- Joomlart: http://www.joomlart.com
- RocketTheme: http://www.rockettheme.com/
- JoomlaShack: http://www.joomlashack.com/

Those 3 are all paid though, but they make good templates. Some free listings:

http://www.joomla24.com/
http://www.siteground.com/joomla-hosting/joomla-templates.htm

Plus you can do a simple Google search for "free joomla templates" and find many more, I’m sure.

2. To find cool plugins like shopping carts, etc, view the listings on the Joomla site:

- http://extensions.joomla.org/

FYI: Joomla already has a built-in user login form. You may need to poke around the Documentation: http://docs.joomla.org/ to learn your way around the interface (if you haven’t already).
References :
Many years of experience with Joomla!

Design your site with what want, custom for your needs. Then plug in modules where you need them to be driven by the CMS system.

I work for a web design company that uses Joomla as our main CMS. We design custom templates for every site… so I really don’t know of a good place to download them.

It is really not hard to make a custom template for you needs. I say look in the default template for the "calls" for the different sections. Like User1, User2, Left, etc. Then you stick those in your layout and assign modules to those areas in the admin.

As far as logins and a database, those are standard, any other features you need (paypal linking modules, etc) you can just search for them online and install them in the admin.

Once they are installed, you can just assign them where you want just like the standard features.

Good Luck.
References :
http://www.google.com/search?q=free+joomla+templates
http://extensions.joomla.org/

I just answered your other question but if you need any…
References :
http://cazpa.com