Something like this:

https://www.amazon.com/Nordic-Naturals-Promotes-Optimal-Function/dp/B002CQU4Z6

Hey man i'm also schizo 20M and like you have had an almost complete regression in symptoms, I attribute this to both medication and supplements(2.5mg zyprexa) which is not even considered a therapeutic dose yet it does the job for me, I typically take it 2-3 times a week and other then making me sleepy I don't experience less motivation just general lethargy. Anyways now for the supplements, i've done extensive research and experimentation and id' say the most relevant ones are N.A.C,EPA fish oil, and sarcosine also throwing in a bcomplex multivitamin with folic acid could indirectly help.

Sarcosine is known to do exactly what you want which is increase motivation, it also seems to help somewhat with social skills which I honestly have not noticed i've seen benefit in motivation though and overall sense of well being. http://schizophrenia.com/?p=740

//N.A.C has been shown to reduce inflammation WHICH IS KEY in getting rid of positive symptoms and helps with negative ones. In my experience this reduced my stress by a moderate amount. http://schizophrenia.com/?page_id=706

E.P.A fish oil has been shown to help prevent schizo, it reduces inflammation, and protects the brain from further damage. In my experience it helped with my overall sense of well being as well as with anxiety.

Now for reputable sources Fish oil:https://www.amazon.com/Nordic-Naturals-Promotes-Optimal-Function/dp/B002CQU4Z6/ref=sr_1_1_a_it?ie=UTF8&qid=1476809252&sr=8-1&keywords=nordic+naturals+fish+oil+epa (I take 2 g a day)
Sarcosine http://brainvitaminz.com/products/sarcosine-180-gram
I take 2 grams of sarcosine a day. N.A.C this one is easier find and purity usually isn't a problem I take 1,000mg, if i'm under alot of stress 2,000mg never exceed 2,000mg

Taking these three supplements have benefited me greatly and have shown to be even more effective when taken with medication. My symptoms have regressed so much that I can take adderall with little to no increase in positive symptoms.

Some other things you should look into Meditation:Just 20 minutes a day split into two sessions

Low inflammation, No grain, low sugar diet (>26Grams of sugar) I'm currently in the process of creating a low cost accessible diet that I can send you when i'm finished if you'd like, infact feel free to PM me, text me/call me what ever I'd be glad to help.