You can order a replacment flap here which includes delivery to common areas. 

Check what flap size you have:

Measure the width of the flap area between the vertical frames of the pet door. Then order the size you have using the selection during checkout of this item.

The medium is 22cm wide.

The Large is 27cm wide

The XL is 39cm wide. 

Tip to remove the existing flap:

To remove the flap, unscrew the nuts on the back of the hinge (do not try and turn the bolt as you may thread the bolt). 

To fit the replacement flap:

Insert the new flap up into the hinge. Put a paperclip or something of a similar thickness under the bottom corners of the flap under the steel clips (if the flap does not come with new clips then re-fit the original clips on to the bottom corners of the flap- they push on by hand) so there is a small gap under the flap so that it does not hit the bottom rail of the pet door. Then centralise the flap in the hinge and in the flap area in the pet door frame. Drill a 5mm hole through one of the existing holes in the hinge and fit the bolt and nut back in. Re-check the flap is still centralised and adjust if necessary, then drill your remaining hole/s. Fit bolts and tighten.