I said they were a "bad idea for general power generation", so I'm not suggesting that they be used to power a Mars colony, but rather that the reasons not to do so don't include Pu-238 being in low supply.
We also have a low supply of rockets & other infrastructure to colonize Mars, but we can simply decide to make them.
Still, I think any extrapolation of current numbers to say that Pu-238 would be categorically unsuitable for such a purpose is probably premature. Nobody's tried to produce it on a truly industrial scale, which would bring costs down. It has a half-life of around 90 years degrading at 1%/year, so once you produce it it'll power the colony for a long time.
It's also around 6% efficient at generating electricity[1], but 100% efficient at generating heat, which is a huge part of energy requirements on Mars when it comes to human habitation. The Curiosity rover generates 120W of electricity but 2000W of heat.
1. https://physics.stackexchange.com/questions/34203/mars-curio...
Pu-238 isn't found in nature, but has to be synthesized. The precursor material, U-238 is the most abundant form of Uranium, which is convenient. It's also produce via neptunium-237.
The real complication is that the nuclear synthesis processes are also those used for nuclear weapons production, which makes for some serious complications.
I'm sure the demand for a colony would scale pretty fast to 100kW or more. Now we are talking about Billions.
So I think that your original point that cost for Pu-238 is not an issue for a colony is wrong.
Small reactors are a much better option as a single ton of fuel could supply a colony with a few MW for decades. You just have to make sure that the largest part is less than 37 tons so it can be transported with a Long March 9.