I have two Acers which are around 13 years old, they are in despair as they need re-potted but we are finding it hard to get a pot bigger than the one they are already in. They are already looking like they are dying and it would really upset me to loose them, is there any way to save them? One has started to drop leaves and I am starting to panic. Any advice greatly appreciated.
Unfortunately, there's not much you can do because, as you suspect, it's likely they've run out of root room. If you can't find larger containers, they can be transferred to the ground, but that is best done in autumn; done now, you'd need to keep them well watered right up until winter begins.
Shrubs and trees in containers cannot stay contained forever (unless they are small shrubs), so they're really only relatively temporary potted plants. You can try trimming back a bit, but that needs to be done carefully so as not to destroy the natural shape. If you do trim back, remove any loose soil at the top and replace it with fresh potting soil and add a basic fertilizer (with an NPK of something like 7-7-7), preferably a liquid one at this time of year if you're in the northern hemisphere because it's really a little late to give fertilizer now. Keep well watered, because the soil to root ratio will be out of balance, meaning there's insufficient unoccupied soil to retain water. Really, though, the best option is larger containers or transferring to the ground.
Some people root prune their Acers every time they become root bound. It involves cutting off about 1/3 of the roots, then planting them back into the same pot. This means you have to be very good on watering, never letting them dry out the first year. Always keeping the soil nice and moist. Not wet.
Winter is the best time to do root pruning.