Hi @ashley11,
Thank you for your question and welcome to the Bunnings Workshop community, it is fantastic to have you with us.
There are shower trays such as this Bellessi 900mm x 900mm Tile Over Shower Tray which can be installed directly to your timber joists with additional timbers for support as shown in its Instruction Manual.
If you wish to install a shower base directly to your joists, make sure there is a method of installation specifically stated in the product instruction manual and confirm with your plumber that this is possible before committing to your purchase.
As the installation of the base is plumbing work, you'll require the assistance of a suitably licenced plumber. I'd suggest you select a suitable shower base in consultation with your plumber to ensure everything is installed to the relevant standards.
Let me know if you have any further questions.
Jacob
