Turbotax entered an amount on line 25c (Federal income tax withheld from other forms) and I have no idea where it came from.

The amount listed on line 25(c) typically represents federal income tax withheld as listed on a 1099 or W-2 form. You will have to look through your tax reporting documents to see which ones have federal tax withheld listed on them to see where that number came from.

Line 25A is for taxes withheld on a W-2, and line 25B is for a 1099.

Line 25C is for taxes withheld as follows:

Form(s) W-2G;
Additional Medicare Tax;

Schedule K-1;
Form 1042-S, Form 8805, or Form 8288-A. 

In my case, the amount on Form 1040 line 25c was "Additional Medicare Tax withholding on Medicare wages".  It came from line 22 of Form 8959 under "Part V Withholding Reconciliation".  It looks like my employer withheld an overly-large amount on Form W-2 box 6 (shown on line 19 of Form 8959).  An amount on line 20 of Form 8959 was multiplied by 1.45% to get the "regular Medicare tax withholding on Medicare wages", and then line 21 of Form 8959 is subtracted from line 19 of Form 8959 to get the final line 22 value on Form 8959.  Again, in my case, line 22 of Form 8959 is the data source for Form 1040 line 25c, which is "Federal income tax withheld from Other forms".

I can't find where 25 C is coming from... I checked my W2 and 1099s and SAs and can't find it. Is there a way for me to check from the TurboTax app where it's pulling from?

You can preview the information on your 1040 form by clicking on Tax Tools, on the left, and look for the amount in the return. 

• Select Tools 
• Then select View Tax Summary, first on the list under Helpful Links.  This will show basic information about the return.
• In the upper left-hand area, in the menu, click on Preview My 1040
• This will show a picture of the information on the tax form and there is a vertical scroll bar on the right-hand side, to scroll down the return. 
  
  Before you file, and after you have paid, you can view/print your entire return and look for the number more easily then. 

I am in the same boat. Go to "Tax Tools" > "Print Center" and click that you want to print not just your return, but also all Federal and TurboTax worksheets. Then use Ctrl-F and search for the number to see where it appears in the worksheets. In my case, it's coming from line 18d, "positive adjustments" on the tax payments worksheet. I don't have any idea what that means, but at least it's a start. (As far as I can tell, the number is completely made up. I had a terrible experience with this buggy software this year, and I will be going H&R Block next year, I think.)

I eventually figured this out, no thanks to turbotax product support. It turned out to be a community property adjustment in my case.