I've been doing some looking around as well. I think your best bet is too become a member on that dwarfpuffer forum (yes there's a forum part of the site), and post a wanted add in the classified section. Some members are successful with breeding and will be happy to send some your way for a low price. (probably just shipping) This way they will be more healthy and younger.
Depending on your specific puffer, you may be able to keep shrimp with them. It's a gamble. Some tear them to shreds others act like they're not there. IMO
, it depends on age and how early they are introduced. You can definitely keep quite a couple ottos with them as well.