The best thing when you have so many pages is to divide the sitemap by categories. If you create an XML sitemap with 15000 it will be pretty heavy and probably unreadable. Try to divide it so that you don't have more than a few thousand urls per page. Of course this is if you are talking about an XML sitemap. In the case of html sitemaps it shouldn't have more than a few hundred links per page and it should also be divided into categories when you have a big site. This way you should be able to use any free XML generator out there.
In wordpress for HTML you can use this HTML Sitemap Generator and for XML you can use this Google XML Sitemap Generator
I think sitemaps are overrated for SEO. I've never had sitemaps on my sites or submitted sitemaps to places like google webmaster tool, and I've always done fine ranking for what I wanted. Just focus on getting more links to those 15,000 pages.