I would say it is better to extract non-edible oil from its sources instead of purchasing, bcoz use of non-edible oil is very limited so its supply also very low. Better u go for oil extraction from seeds using a simple technique (Soxhlet) that will minimize the cost of raw material and ultimately of biodiesel.
It depends on which seed u want, I will suggest you find the seeds easily available in your area. It will be better if you find a seeds available as a waste in the local area. However, non-edible oil like neem oil can be easily available in local market.