How can I find my old PCS Orders?

If you don't have them then they're gone. The Army cuts millions of sets of orders a year. Units are only required to keep records for two years for most things and destroy them after that. If it's been more than two years, the MPD that cut those orders has probably deleted them by now.
Lesson learned, keep a copy of everything important until you're sure it's no longer important. PCS orders aren't usually that important after you PCS

This doesn't really help you now, but you should have been saving and safeguarding your own documents whenever you got them. That's an individual responsibility.

ARIMS only requires personnel documents to be retained for anywhere between 1 and 6 years, depending on the document type. In my S-1 Shop, we shredded everything older than 2 years, to both comply with ARIMS regs, and because there wasn't enough room in the file cabinets to retain personnel files of everyone that came and went through the battalion.

Orders are not usually iPERMed by the units. I have copies (both digital and hardcopy) of every PCS, TCS, TDY, and skill/MOS order that I have ever been given. This way, I won't ever be in your situation if/when anyone ever asks for a document. Do you have an "I Love Me" Book? If not, why not?

If it orders sending you where you are now, see company clerk. A copy would be in your field 201
File.