replacement for a NDP6020P. They stopped making them. How do you find an alternative?

It seems the part is obsolete and Newark doesn't carry it anymore,  but there are other places that seem to have stock.

https://www.solarbotics.com/product/17174?countrycurrency=CA&gclid=EAIaIQobChMIz9HRsN2j9wIVIWxvBB2XVgM3EAQYAiABEgLTtvD_BwE

A part that should work and is available at Newark is IPP80P03P4L04AKSA2

thank you i picked up  a few i think this will be a part i will need a lot with what I'm doing.

Hi,

You might want to share the circuit that you plan to use this device with. It's up to you if you wish to do so, but if it is a hobby circuit from the Internet, it may well not be ideal (there can be better ways to achieve things). RIght now I don't know either way, maybe it actually is the perfect ideal part, but I have no idea, and just from being an overly suspicious person usually!, I'd be wanting to examine the circuit if it were me.

this is what I'm doing. It needs to have a button large enough for a service dog to press and require as little charging as possible. This should shut off 10-15 seconds after pressing. It sends a message to home assistant for emergency alerts. I am very new to electronics (less then a year). so I don't know if this will work till breadboard.

This circuit doesn't appear to need a big FET, almost any P-FET with a low gate threshold would work, although a low on-resistance is always nice. 

You may want to add an NPN/NFET in parallel with the pushbutton. For the WeMos to turn itself off it needs D1 to rise to 5V(or vbat?), the ESD diodes on the chip may prevent that from happening. Neat project! 

Also, as well as the above comments, it will be worth really inspecting closely what the modules you are using do on the pins you're connecting, because I can't see that the 1904 LiPo charger has a 5V output. There is only the battery voltage output. That's important to know if that is the case, because if you connect that to the pin on the Wemos board that is labelled 5V, that's incorrect because that pin is really called VBUS, meaning the voltage that is on the USB port connector. It will function, but if anyone ever plugs the USB connector on the Wemos to a PC, then the battery will see 5V across it, which is dangerous. There's no quick fix here, so it needs some thinking through, You could try to hack the Wemos board so that the 5V from the USB connector is disabled, e.g. with a pin header with jumper, so that if the user ever wishes to plug in the USB connector, they would need to un-jumper the battery connection first. I have never used a Wemos so I don't know how hard/easy it would be to make such a change to the board.

I'm very new to electronics and only 1/2 way through Make: Electronics by Charles Platt. So my knowledge is very limited. The idea is that as soon as it boots up pulls D1 low to keep itself on. Then it connects to the network and sends the mqtt. The last line of the code "digitalWrite(D1, HIGH);" should kill its own voltage turning it off. At least that is the idea.  As far as "NPN/NFET in parallel with the pushbutton" I honestly have no idea how to do that.

Something like this basically.

The NPN can be (say) 2N3904 or BC547. 

The USB connector has 4 or 5 pins on it, Wikipedia has the pinout for USB, and if you can isolate the VBUS connection (I've mistakenly labelled it VCC in the diagram, but it's VBUS) then you could add a jumper as shown.

With the jumper shorting pins 2+3, then the LiPo cell is used to power the Wemos module. With the jumper shorting pins 1+2. then the USB connector on the Wemos module is used to power it, (e.g. when programming it).

Without the jumper, there's a risk you'll forget to unplug the LiPo cell and plug in the USB connector on the Wemos module, which will effectively place the 5V from the PC onto the battery.

Good point. The wemos board will be inside a case. so no one will be able to connect the wemos usb into anything. But now I wonder if the 1904 will do the job. You are right a lot more thinking. The bat only has 3.7v and min power for the wemos is 3.3. that might be to close. Maybe I need a 9v and convert it to 5v. then just change it every year. I just hate to waist.