Felix sells new ones at the bazaar. I've lost a few and sometimes they show up again in my storage... I have no idea how that works but its happened a few times.
Buy a new bone. Felix sells it just as the people say ._. dam im late
I've lost that damned bone too... Just buy another one and you'll be fine! Careful not to lose your dog bones again, ok? x3
The bell can end up in your storage if your kitty knocks it out your front door at a certain angle that lines up with the shipping basket that is linked to your storage.
(the bell is "put" into the basket thus ends up in storage)
There's a glitch of sorts where the bell dissapears if it goes off-screen... same with the bone if I remember correctly.
Also, Mr. Mayor is random in what he sells, so just wait for next week (because I assume it's too late to reload your game). He'll sell it eventually...
Sadly this has happened to me and I read that it will sell again (randomly) by Felix at the Bazaar but I've been waiting for THREE weeks and he's STILL not selling one *grumble * *grumble *
(been waiting same amount of time for Chen to sell Rice *grumble *)
what you can do is Friday night go to bed and save, then saturday on the bazaar just go there and see if he's selling it. If he isn't, then turn of your gameboy and restart it. Check back until he's selling it, it'll happen eventually.
* *grumble 
o for heavens sake, you know what she means... lol. i still call my ds a gameboy too. it is just what we grew up with 