what can i do to stop rainwater getting under my garage door

i am planning to turn my garage at the bottom of the garden into a small workshop. its one of those old pre-cast concrete monstrosities with a corrugated asbestos roof.
we had new wooden doors fitted but do not actually intend to open them regularly as another side door gives me adequate access. we have since discovered a problem.... the floor of my garage seems to have been laid on a slant than is an inch or so lower than the road outside the main double doors!
every time it rains water runs under the door straight to the back of the garage where it collects. unfortunately i cannot afford to have the floor raised up (the builders have long gone as it was built in the 1930s) so i was wondering if there is anything i can fix to the bottom of the doors that will keep the rain out? some sort of flexible rubber flashing for instance.
any ideas would be greatly appreciated.
many thanks davidafield

See the query by Mike004, he's got rubber from wickes for the bottom of garage door, but I can't see how it would make it water tight.
The only way I would think is to raise the entrance by 2 in with small ramp either side if you still intend to put car in it.

3 things

1 live with it

2 make a new floor in the garage (my brick shed has some timber soaked in ? on the concrete floor)

3 Put a gulley and soakaway in front of doors