Vegan

There are some supplements containing DHA but little to no EPA. These are a very bad choice, because EPA is essential and the body can't convert DHA to it.

Conversely, there are almost no supplements with only EPA but no DHA (that's probably because EPA is more expensive/rare than DHA). If there were such supplements, these would be a semi-good choice, since the body can convert EPA to DHA, albeit only to a limited degree.

Since most supplements tend to be DHA-heavy, aim for supplements with EPA and DHA about equal. Whenever they only tell you the total Omega 3 content, not broken down into EPA and DHA, assume a preponderance of the cheaper DHA or even ALA, and thus avoid it.

Price-wise, offers are hard to compare, since most make up their own "daily dose sizes" etc. Thus, I highly recommend making a spreadsheet like the one I recently did here for offers in Germany. Making this spreadsheet hardly takes more work than doing the comparison in another way, and you can then share it here to save other vegans who live in the same market area the same work. A sensible column layout for the spreadsheet is like the one in the example:

Name (with hyperlink) | Price per package | Number of capsules | DHA per capsule | EPA per capsule | (DHA+EPA) per capsule | EPA per € | (DHA+EPA) per €,

with the last three columns being calculated from those before.