I have just started my first artillery project. It is a Tamiya pak 40. I would like to get some photo etch parts for it . Who would be the best to order from ?

Aber and AFV Club both offer photoetch accessories for the Pak 40, though they are intended for the AFV Club kit, which is 30 years newer than Tamiya's version. Great Models webstore carries them. They are each around $11-$12. Aber also offers replacement gun barrels in turned aluminum and brass. Griffon offers photoetch and barrels for the Dragon kit. I don't think there are any upgrade kits currently made for Tamiya's due to its age.
Alternatively, you can get the recent Dragon kit with crew figures and photoetch included for around $20.

thanks for the info.