A professor or serious student of philosophy will likely have a substantial personal collection of books accumulated in the course of their study or research. A university may have a small departmental library for philosophy in addition to resources in general or other libraries. A university library system will also likely provide affiliates institutional access to JSTOR and similar online resources. They may also have print subscriptions to periodicals and journals.
Exactly which resources are valuable will depend on interest and field of study, and you probably want to ask a relevant expert to save time. Journal access is very important if you're doing active research, but is less important to more casual philosophers. The internet and SEP are very good, but it's important to have other resources available in case they don't have enough information or you want a different perspective. There are actually various encyclopedias of philosophy (and certain subfields) with different strengths and weaknesses, as well as dictionaries and handbooks. These are generally good investments for an institution, but an individual will want to be prudent.
For general knowledge in a particular field, a student of philosophy may want to invest in a relevant handbook or similar resource. These are designed to give a basic foundational overview of some important ideas in the field and I think many are such that graduate students in philosophy could be expected to understand with little help. Oxford University Press has a series of these, as does Routledge, but the relevance and usefulness will depend on the individual volume and for certain topics another publisher or resource may be better.
For more detail, you might look for a reader, compendium, anthology or similar work. These will contain primary sources or excerpts which will let you engage directly with a relevant philosopher's views. There are a wide variety of these and many are selected to be suitable for undergraduate courses. I think academic lectures and discussions, the internet, encyclopedias, handbooks and readers are the main sorts of resources (secondary sources) which are useful in philosophy but aren't likely to be seen cited in a paper.