Question: What type of conduit can I use to run 50' of 8/3 cable from main breaker panel to spa sub panel along the outside wall of my house to get a 240v circuit to my Spa?
Let's start with your options. First if you are running underground it would be PVC or direct burial cable type UF. Second if you are running above ground it would be any conduit system approved for such use. The most common PVC, Emt, nonmetallic flexible conduit commonly known as Carflex, and pvc coated metallic conduit commonly known as Sealtite. Or any combination of the above.
Personally I would run EMT with WT stainless steel connectors and couplings. Then transfer over to some type of flex to connect to your panel. Also I would suggest that if you are running conduit I would use 3-#8 w/ #10G THWN or Xhhw conductors instead of an 8/3 NM cable. It would be less expensive and since you are running conduit no extra protection is needed.