Seems odd that the only rings you can get for your cranks are 53/39. If you talk to your LBS I am certain that they will be able to source rings in the teeth size you want. On my Mountain bike gearing I can go a lot lower, but that is mountain bike gearing- or is it? I run a triple crankset- and use 48/36/24. I can get rings for my large ring up to 54- middle up to 42, and that is on an obscure Sugino crankset. The variety of rings made by Non original manufacturers is enormous. It could be that you have a crankset that cannot go much smaller, but have you thought about changing the rear Cassette? Could be opening another can of worms with new derailler being required, but Talk to your LBS and see what is possible.