I was wondering if anyone had information on the availability of small liquid fuelled rocket motors in the US (or elsewhere)
Moog offers several such thrusters:
Rocketdyne has an entire range of thrusters as well:
In Europe, there is always airbus:
This is a very strange question as far as strange questions come, so let me answer is straight away; I have not found any resource online that sells liquid fuelled rocket motors.
Now even if I had, the legality of a 500-10,000 Newton rocket motor is very shaky. According to the FAA Code of Federal Regulations (paraphrased):
If the rocket & payload (including fuel) weighs more than 3.3 lbs... If the rocket has more than 125 grams of fuel... If a single motor rocket has more than 160 N-sec of impulse... If multiple stages add up to more than 320 N-sec of impulse... If the average thrust is more than 80 N...
... you will need a waiver from the FAA to launch the rocket.
Any rocket exceeding these parameters is subject to approval on the FAA's discretion. There are a host of other complications like proximity to population and airports.
To conclude; there are far better (safer) propulsion systems than liquid fuel motors.