Hi there,
We had the same dilemma when we started designing our prints. The Ikea in UK sell Ribba 30 x 40cm which is also quite standard in frames and they aren't too hard to get hold of. We designed our prints to be 20 x 30cm so there was a 5cm border all the way round. A mount could be added but not fully necessary as the white acts as a bit of a mount anyway.
Hope that helps a bit!, all the best, Jasper