I have Big Softies as well, I got mine from Baby Bunting. I agree with Sammi, you should be able to get them from any of the baby stores.

I used flannies until DS was about 4 months, then changed to TT. I use a snappi now, but didn't use anything with flannies, just made sure the cover/snib was on nice and firm!

If you haven't already, check out the eenees snibs, they are great to use with flats.