To start with I suggest you buy a thread gauge, vernier gauge (digital is easier) and thread identification tables (which you can download free online).
Once you have these you then need to learn how to use them to identify what type of threaded fixing you need and the terminology for them. Then it’s pretty straightforward to ask for the specific part online or from specialist suppliers like Namrick.
Past Parts can repair brake cylinders but you can also get new ones. I have just bought a pair of front cylinders for £70 inc p&p. online. I just googled Allard P1 front brake cylinders.
Note that your P1 will most likely have the Lockheed phase 2 brakes. Earlier cars had phase 1s which are not as efficient.