Lab ideas for Cisco ASA firewall?
I have been asked to master Cisco ASA as soon as possible 

 and  my work environment requires such knowledge [intermediate to advance knowledge of Cisco ASA].
Question for All Cisco ASA Expert:
How/where did you practice your skill and what material did you use?
I did some research on home lab idea, but the cost of  used Cisco ASA 5512-x or 5515-x is untouchable. I also tried to run Cisco  ASA 5520 using GNS3! Very unreliable... crashed all the time [at least that is my experience]
Any recommendation for a reasonable price rack rental company/org is much appreciated.