Calm down and try not to breathe too much. I've avoided many a milder asthma attack by that method, but years of practice mean I'm fairly good at not breathing. YMMV.
A friend of mine used to swear by having a wank. She was prone to getting into a panic cycle that would exacerbate the problem, thobut. The opposite effect seems more likely to me.
More importantly you need to get away from this "my inhaler" idea that the penny-pinching medical establishment like to perpetuate. If you're asthmatic you should distribute salbutamol inhalers liberally throughout your life, so that you're never too far away from one at times of need. So there's an inhaler in your handbag, an inhaler by your bedside, an inhaler in the kitchen drawer, an inhaler in your cupboard at work, an inhaler in the glove compartment, an inhaler as part of every bike's toolkit, etc. You do then have to keep an eye on expiry dates, but I'd much rather have a slightly out of date inhaler than not have one at all.
