Bike Sizing Fit (before buying a bike)

Hi - I am looking to buy my first proper road bike for semi-competitive use (triathlons etc.)

I have been getting confused about whether I need a proper bike fit before or after buying.

It sounds like it’s worth doing a sizing fit to make sure I buy the right size bike. Is this right or is it actually pretty straightforward to buy the right shape bike? If so does anyone have any recommendations for where to get a sizing fit.

I know afterwards I will likely then want a proper bike fit once I have ridden for a few months. Thanks very much!