The viablity of this is really down to price.
In the UK fuel is so expensive that people actually buy drums of new
vegetable oil to run cars.
I have even seen someone going round the supermarket filling a trolley with
plastic bottles of oil when its on special offer.
I would guess that if veg oil costs not much less than fuel in your country
then you would need to use waste oil to make it worthwhile.
Waste oil needs to be filtered.

I would recommend doing a search at www.google.co.uk and specifiy pages from
the UK
Look for biodiesel and simialr search terms.
There is lots of info out there or try uk.rec.cars.misc

Well, there are alot of choices. If you want to do the whole process
yourself, it is the matter of just asking the owner to take used oil from
them. Majority of them pays someone to remove it for them... not much money
though but some vendors are so stingy they still want to charge restaurants
more money.

Restaurants use oils regularly so they also have tons of those 4 or 5
gallons containers they throw out... you can ask them to put them aside for
you or if they will simply pour them into the container for you... perhaps
you can supply them widemouth funnel to make their job easier. Otherwise,
you just need a pump to remove it from the tank... only skim the top to get
the good oil... bottom are water and gunks. So it helps to let the oil
settle then ask them to put it in containers for you.

Other choices is simply buy the strained WVO from other people... like in
Connecticut there are biodiesel coop... where people share a tank... one
person handle all the work and you pay... like a dollar a gallon. Biodeisel
coop also buy biodiesel from  commercial supplier but they cost $4 a
gallon... yet, the plus side is no OPEC oil... all vege and animal fat
processed.

There are vast resources on internet... just look into it.