Welcome
As I understand it, the Fiat/Alfa market in AU is a bit lacking in variety, so finding 4x98 alloys from other Fiats which will fit yours will be a pain.
While the 4x100 options are not a direct fit, there are ways around it but you have to take into account more factors than just the PCD. Wheels fitting rules revolve (sic) around:
- PCD
- Centre bore
- Offset (ET)
- Diameter and width (ex. 15x6J is a 15" wheel with a 6" width)
- Rolling radius (of the tyres)
PCD can be corrected with PCD correction (wobbly) bolts. While some question the safety of anything which is sold with the word 'wobbly' in the name, these are perfectly safe as long as you don't buy cheap ones and the wheel can't move around on the hub. This is where the correct center bore is essential. Fiat use a center bore of 58.1mm, so you can buy alloys which have a value higher than this and correct it with spigot rings, but you can't - for example - use GM group wheels, as these have a center bore of 56.6mm and so will not physically sit on the hub. The center of the wheel must contact the hub properly (ie no gap) or you run the risk of the wheel wobbling about and coming off.
Offset (expressed as ET ie ET35 or ET-15) is how far the mating face of the wheel (the bit which touches the hub) is from the center of the wheel. Negative numbers being more inboard means that higher offsets can be corrected with spacers (so if your car has an offset of ET25, and you buy wheels with ET35 you can correct this with 10mm spacers) but remember that you will need to make sure your bolts are longer to accommodate the extra depth. Offsets which are too low cannot be corrected.
Diameter, width and rolling radius and fairly self explanatory (I hope, because I'm bored of typing now!).