The System map is your friend here.
Use the galaxy map tools to find algae moving around and note the source as suggested by others.
Click on the source and go to the info tab, if you know about the system the economy will include Agriculture.
If the Info does nto show this data you do not know about the system. I think if there are stations, tese will show up in the system map.
Othewise you might have to buy the system data.
In the system map click on the station will give you brief import/export and economic type - looking agriculture for algae.
Make sure there is a commodity market at the station!
Personallly I always like to know about a system before I jump there (unless I am exploring!). LHS 3447 is a starting system nice mised agriculture and (refinery I think) economy, but I only go back there if I have to, 100,000ls to the stations from the entry point in the system. The system map shows you the different stars etc. and orbiting distances so you can make a decision about needs verses long journey time.
The multiple commodity markets and stations with different economies was new in gamma 2, through a lot of out at the time - exactly as multiple stations in a system has for you.
Can't you also use the system map once you get to that system to see which ports/docks export that specific good? I haven't actually looked into it but I think I remember seeing that.