Some of the stuff that Sony makes are specifically for iPods. However, Sony does make stuff specifically for the Walkman, so you should try there first. The bottom connector thing is proprietary, so it might be difficult to find third party accessories that work with this connector rather than the headphone jack.

Here's a link to the SonyStyle online store. They're not cheap, but they should work.

http://www.sonystyle.com/webapp/wcs/stores/servlet/CategoryDisplay?catalogId=10551&storeId=10151&langId=-1&categoryId=8198552921644515824