Can I get TurboTax 2018 before 12/31/2018?

It's usually released in mid-November.

If you enroll in Turbotax Advantage, you will get the earliest possible notification of the availability of the program (to install on your own Mac or PC).  There isn't really a price break, but you get both Mac and PC versions, a digital download and courtesy CD copy, and access to a priority support phone number. https://shop.turbotax.intuit.com/advantage/

The best price for the download or CD version would be wait until January and buy it on sale at Amazon or Costco.

If you plan to use Turbotax Online, you have to wait for them to turn it on, usually late November or early December.

But you cannot file until early 2019.

And the program is not fully operational until mid to late February at the earliest.  This year there are not only the tax law changes to deal with the IRS had revamped the form 1040 which will require a major overhaul of the income tax programs ... so don't expect anything to work well right out the gate.

If you need to estimated your 2018 taxes then you will use the 2017 program that is operational now.

and many states are still considering whether/how to change their returns as well

I'm not looking to file early, I just want to make sure that I have enough withheld as I pull cash from my IRA.  My goal was never to get a big refund, but to "owe" about $100.

Try these tools:

<a rel="nofollow" target="_blank" href="http://www.irs.gov/Individuals/IRS-Withholding-Calculator">http://www.irs.gov/Individuals/IRS-Withholding-Calculator</a>

 <a rel="nofollow" target="_blank" href="https://turbotax.intuit.com/tax-tools">https://turbotax.intuit.com/tax-tools</a>

<a rel="nofollow" target="_blank" href="http://turbotax.intuit.com/tax-tools/calculators/taxcaster">http://turbotax.intuit.com/tax-tools/calculators/taxcaster</a>

"pull cash from my IRA" is it an RMD?

You can always make estimated tax payments.