This is the wrong section to post this.
I wouldn't say I have any 'good' collectibles, but when events are around I sometimes buy one with that currency or they're on sale at some point. I don't really care about collectibles nor about BTB, so I might use some to get a icon for my profile.
We've been saving up for our TBT, getting lucky in restocks. Pretty much what the other users said. That's how some of us got so many. Maybe you can get a lot too.
And btw, most collectibles are one time. So if you missed out on them, you have to buy them from others, or winning giveaways.
Participating in events, getting lucky in restocks, stalking TBT Marketplace for sales and giveaways, and saving a lot of TBT. I've spent very little on most of mine, getting most of them from friends for cheap, or from sales, giveaways and restocks.
Oh, and you can sell your soul for them too. You can get quite a lot