Same here in the Netherlands - and the Transcend model recommended by Danny is not readily available. However, the text in the Roon KB implies that there shouldn’t be an issue using a larger alternative:
The SSD can be 64GB or larger – note that larger SSDs tend to be faster due to how the chips are laid out (after 256GB, it doesn’t matter… for now). It is extremely rare you will need more than 64GB of storage on this SSD. ROCK will not use it.