So the game just released for us, so there's not a lot of parts in the personal shops for now, Is there a way to obtain parts from the past 8 years or we can only get what's coming forward with the tickets?

They are releasing various sets of parts from the past 8 years a few different ways. Some are being released as part of the AC Scratches, though they are primarily any that were released from Episode 4 and later; others are part of the Gold Mission Pass, and these have been from Episode 1 to 3, but they are account-locked; a few Female CAST sets have also been released in the Fresh Finds Shop, which is SG only and account-locked.

CAST parts shown on the AC scratch news posts are the items we can still have some chance to buy on personal shops in NA. (https://pso2.com/news/scratch-tickets?page=1)

FUN scratch doesn't have CAST parts, so you can ignore them.

I suppose body/arms/legs parts are easier to buy, but the tickets for the head parts can be expensive, or worse, simply sold out.