How do I determine my total Special District purchases for 2023?

Without documentation, there's no way to nail down the actual number. There's not an estimation calculator for computing this, at least not in TurboTax. 

You can make an estimate, going through credit card transactions and bank statements.  That would be sufficient, as long as you're reasonable.   The burden is on you to keep your receipts and report this tax.

That said, according to 1. Am I required to complete the consumer use tax portion of the individual income tax return? on the Colorado Department of Revenue's Consumer Use Tax | Frequently Asked Questions page:

Leaving that portion of the return blank indicates the taxpayer does not want to use the income tax form to report and remit any consumer use tax liability they may have for the prior year. However leaving it blank does not mean that the taxpayer is formally stating there is no use tax liability. Taxpayers may confidentially and without fear of criminal prosecution report zero on the use tax line of their individual income tax return. The Department's working assumption will be that the taxpayer either had no use tax liability or they chose to report it through one of the other methods available to them.