The Chinese company MHW-3Bomber makes a good Breville 54mm stainless bottomless portafilter for ~$45. You can get it a little cheaper ordering from Aliexpress.

https://www.amazon.com/dp/B09W2F6Q37?th=1

If you are saving for a better machine, I'd save the money and get one of the cheap zinc alloy portafilters. As long as it fits, stainless gets you very little except long term durability. You can get this one for $10. If it doesn't fit just return it and try another one.

https://www.amazon.com/Bottomless-Portafilter-Compatible-Replacement-Accessories/dp/B08N6BCMT9/