I thought about trying to make my own so I always had some on hand but since I want them to look as original as possible I don't want a dodgy Aaron-made bubble ruining the card.
I've done a bunch of these in the past - a tool that I finally invested in that you might find useful is a round-corner punch (pic attached)...I think I picked one up at Warehouse Stationery. It'll give you perfectly round and perfectly aligned round corners every time.
I don't know if they had that corner punch (or a similar one), but Warehouse Stationery has a sale on at the moment - there's 25% off Friskars 1" Lever Punches and there was a pile of punches in one of the bargain bins at the Wairau Park (Auckland) branch this afternoon.