Unclebun is right. You can make your own easily. We usually use either coroplast or foam core to make our easels. Just use a small cardboard easelback (attached image) as a template for the angles, score the straight side that makes contact with the cutout (an inch or so should do), fold back the flap and adhere to the cutout with double-sided tape. I found a nice image that you can copy and scale and attached it here. You'll want the easel to be about 2/3 the height of the cutout and you can always add a second one if you're having a really hard time.
Also, 3mm is definitely going to be floppy. We usually use a high density foam board like Gator or Ultraboard for interior and coroplast or ACM for exterior depending on the client's budget and how long they need it to last. Hope this helps.