I have Vegas Pro 18 and I would like to know if it is possible to buy/download new effects or transitions for it for free - and if so where can I find it?
BorisFX, NewBlue FX, proDAD are paid OFX plugins. I don't understand your question though. You want to "buy/download new effects or transitions for free"?
Vegasaur provides some free effects and transitions:
https://vegasaur.com/download
The more interesting - and more elaborate, of course, are for sale.
After all, we all need to earn money to buy something to eat.
Vegas accepts several types of video and audio plugins: DirectX, OFX, VST. Thousands of each are freely available for download on the internet, although all are not tested in Vegas before they are released.