A large university library might be the best source – they will have the actual copies of many journals, and probably subscriptions to a lot of online services that you’ll be able to printout (although the printing may cost you).

If you have to travel any to reach one, though, call ahead to find out what their policy is for non-student usage. Some don’t care (as long as you’re not taking anything out), some have agreements with your own library (assuming you’re in school somewhere) or will make other provisions, and some will stop anyone without a school ID from entering.

If you can not find any PDF on line google should turn up a few (type in a subject). Try a Library they usually carry some scientific journals. If you are close to a university they defiantly will have some.

A list of scholarly journal with free acces to their text is available on http://www.freemedicaljournals.com/
There are a score of different policies varying from completely free access (All the PLoS journal, BMC,…) to access granted after a certain time (usually 6 months to a few years).
And Elsevier the biggest publisher has a very simple policy: no free access (curse them), so you’ll have to go to a library with the correct subscriptions.