Controllers by Curtis, Kelly and Zilla are available fairly readily. The former can be bought in Australia, which can save a lot of hassles. There are others, too, but you will probably have to track them down and import them yourself. Consider the backup if you need it!
Another system worth considering is the D&D separately excited motor/controller pack (SepEx). This is a 6.8”dia motor which is 11” long (similar to the K91-4003), but, instead of being series wired, the armature and field are independently controlled by the controller. This allows for the torque to be optimised at low revs, while increasing the speed at the top end.
The SepEx system comes as a tuned pack and uses a Sevcon SEM PPAK controller rated at 425amps. The voltage range is 60-84volt. It appears that they are only available by importing yourself.
Performance is similar to the ADC K91-4003, but it automatically comes with regen braking.
If you are building an EV under 750kg, this system is definitely worth considering. To date, there do not appear to be any larger SepEx motors available, but they will probably come.
At the time of writing this, the AU$ has been in free-fall, and the prices of the recent past will be but a sweet memory.
When the AU$ was up within 5% of the US$, the prices went something like this:
K91-4003 about $1500
X91-4001 about $1700
L91-4003 about $1700
203-06-4001 about $2500
FB1-4001 about $3400
Alltrax AXE 7245 about $900
Curtis 1231C-8601 about $2500
D&D SepEx motor & controller system, about US$1450 plus
freight, customs, etc (allow about AU$2200)