There are plenty of xml sitemap generators out there. It all depends on how much you want to spend. If you have multiple e-stores, I would say, find one that has no restrictions on domain use.

I have OSC/CRELoaded e-stores, and utilize an sitemap creator from magneticone.com - very easy to use, inexpensive, and I can use across multiple domains.

It also depends on what platform your stores are running. If you have a wordpress, or joomla store, then it's just a matter of downloading the correct plug-in or component for each.

Another usefull one is seomaps.com - which is run locally on your desktop, then it's uploaded to your server via it's built-in ftp engine. Very easy to use. Just follow the instructions and your off!

Hope this helps some.

- Davis.

There's bunch here:
SitemapGenerators - sitemap-generators - Google Code - A collection of links to tools and code snippets that generate Sitemap files.

I personally think it's not necessary to put all your pages into sitemap. Especially for those huge inner pages.

try using software

Definitely it would be better to use Sitemap Writer Pro.
It does well over 1.000.000, I'd rather say Unlimited number of URLs. Try it, you won't be disappointed.
Official website-sitemapwriter.com