Where online do I find the customs form?

Here you go.
https://www.inm.gob.mx/spublic/portal/inmex.html

Logging in here is consistent. Aduana at ZIH is hit-and-miss.

Just a friendly reminder that flying direct from Tijuana to Zihuatanejo means you won't go through customs in Zihua. If you get someone to drive you in a car across the border from San Diego to Tijuana, the chances of being randomly stopped are much less than 1%. Just ask any gun-running cartel mule...

In the meantime, you can stay a few days in the Tijuana / San Diego area. We have plenty to see and do. A favorite crowd pleaser is the Valle de Guadalupe Mexican wine region.

I have breezed through many times as have friends. Then there are the other times. One time I thought a friend had missed his flight because he hadn’t come out almost 10 minutes after the last passenger had left the airport. The airline counter confirmed he had been on the flight. Aduana was making him pay duty on an old, used Apple TV for which he didn’t have a receipt. He (actually I) paid more in duty than the value of the ATV.

Then there was the time I brought an old blender with me. I too paid more in duty than the value of a used blender.

It seems that when I bring new things and have the receipts aduana doesn’t even stop me. I just go straight through. It is when I don’t have receipts because I’m bringing old things with me that aduna goes through everything I have..

It’s actually the immigration agents. (The ones you present your passport to) They will ask you how long you are staying and enter this in your passport. Make sure this date is accurate. Then you go to customs with the filled out form that you either got on the plane or was handed to you.

I have a couple of modifications to this comment from discussions with people I know who have visited during the last 18 months.

1. The immigration person might ask how long you are staying. Or they might not. Some automatically enter a number of days. It might be 180 (the maximum for tourist entry). It might be 90 days. It might be some other number.

2. You don’t want to get stuck with a duration shorter than your stay.

3. To prevent problems, I advise friends to either tell the immigration person how many days they want (without waiting to be asked) or to put a sticky note on the form requesting a number of days.

4. I advise them to not make the number of days accurate. If they are planning to stay for six weeks, I tell them to ask for eight or ten weeks. I don’t want them to run into immigration difficulties because their flight was delayed, or they had an illness or injury, or because they fell in love.