[spiderserie4]

What year Alfa?

exactly

...we try to help on the BB, but....

 

[spiderserie4]

can't believe you're still driving around 2 months later with no cap on the fluid reservoir!
over here they are available just about everywhere for under 10 bucks
originals as illustrated here made by ATE. Shows PN and sizes too if that's a help...

 

[spiderserie4]

no breaker's yard near you?
I bet a ton of german cars (BMW, MBZ etc) will have that same cap....they all have ATE brakes, and that cap is ATE