|You can make your own 'box style' envelopes from regular ones.
Take an envelope that is larger than your piece by at least twice the depth of the piece (your piece is 4 x 6 and 1/2" deep, you need an envelope that is at least 5 x 7)
Around all 4 edges SCORE a line that is 1/2 the depth of your piece from each edge (On the sample above that would be 1/4")
Now fold each side of the envelope IN so that the scored lines are on the outside and the original folds are tucked inside them (like the sides of a paper sack)
Take the little corner triangles left at each corner and tape them to the borrom of the envelope.
These measurements will be a close fit. If you want your piece to have a little breathing room, increase the overall size of the envelope and make the side creases a little further from the edge.