What area are you in??? there are a few specialty pond stores around the country.
for liner there are a couple of options, there are some good quality fibreglass and Polyethelene preformed ponds.
if you are going freeform I would strongly recommend the Firestone liner, its rubber so it stretches without tearing, (I have seen a truck driven over it on a gravel driveway without even a scratch or pinprick) and its UV proof (10 year UV garantee)
Pressurised canister filters are extremely useful and should be considered, but if the pricing is too high you can always add to it later.
imo pumps are quite important, unless the pond is extremely planted and shaded and water movement is going to help.
also using a rivverstone in the base will increase surface area for bacteria to colonise and help keep your pond clean.