I found my blank bag at Michaels for $7 (then used a 20% off coupon) and two bottles of fabric paint (Tulip brand).
The result? Not perfect, but not too bad. I think the problem was that even though I used the mat to keep the bag flat, it didn't work 100% and so it was difficult to get the paint through the stencil and on to the bag. Maybe if I were to do this again I would iron the bag first.
Have you ever tried to stencil a fabric bag or anything else? Have you ever had less than favourable results?
I linked this post up here: