I'm trying to reference this data for an undergraduate course, but I'm not really sure where to find it. I have looked for something I could possibly find through literature, but can't find anything definite. Is this data something that is available through a database?
Take a look at the GTEx database
You can search for your gene in Expression atlas, where you can see its basal expression and it's expression in certain experimental conditions.
Some Expression data is available in NCBI Gene as well. For example, search for human apoe in the NCBI Gene portal and navigate to the APOE gene page where you will find a bar graph of expression data in the 'Expression' section. You can choose one of the four different large scale studies to view their data by clicking on the drop-down menu at the top left corner of the 'Expression' section. Finally, you can get further details such as RPKM values, read counts, links to SRA and BioSample, and the ability to download the data by clicking on the 'see details' link at the top right corner.
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version 2.3.6
You might try filtering/searching GEO datasets for the different ApoE isoform combos, though be careful averaging across datasets given their inherent differences in experimental design and approach to quantifying ApoE isoforms.
e.g https://www.ncbi.nlm.nih.gov/geo/query/acc.cgi?acc=GSE102956