Last minute scheduling can be difficult if you are not easily verifiable. I wouldn't necessarily say I consider it bad etiquette, personally. As long as you have references who respond quickly, or other easy ways of verification, and I have the time in my schedule, I have no problem with last minute appointments. Just my two cents..

Explain your situation and it might give you a better shot at getting in last-minute with someone because they already have you screened.  No guarantee of course that they will have an opening but it's one tactic.

Also, there are providers who usually don't schedule appointments in advance; it is first come, first served.  But be aware that some of those will book more than one client for the same time and whomever shows up first gets to see her.

I think the easiest way to see someone reputable last minute is to see someone you have seen before. You might still get some no's but if she has time, and already knows you, she might be more likely to say sure come on over.