Spaceship Part #21
¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯
This one is well hidden. First of all you need to go find the Land Act Dam which is just northeast of Los Santos. Now, in the water portion you can find a dock. Head down to it and jump in the water. Now dive and search under the dock to find the part.
Spaceship Part #22
¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯
This one is fairly close to the last one. From the Land Act Dam (which once again is just to the northeast of Los Santos), drive up the west road until you see a building with several large pipes heading down to the water. Go inspect the pipes to find the part at the end of the left one.
Spaceship Part #23
¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯
This one is pretty simple. Head to Lake Vinewood (North of Vinewood, of course) and head out onto the dam. Head to the central support column. Now, jump into the lake. You will find the part on the lake bed near the central support column.
Spaceship Part #24
¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯
This one is a little tough to find. To start, grab a car and head for Route 15 (the Palomino Freeway), on the east coast. The road has a big curve to the right as it goes northward. At the end of the curve (before the road starts heading left again), get out of the car and head down to the coast. For a reference, the "Palmer-Taylor Power Station" should be north of you. Now that you are on the coast, head north and soon you'll run into a large rock arch. Swim under it and check to the left to see the part chilling on the beach.
Spaceship Part #25
¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯
This one is pretty easy. Head to Route 11 and Marlowe Drive, then head east down Marlowe Drive. Keep your eye open for signs shortly after heading down the road. You should find one that says "Parsons Rehabilitation Center". Head into the gate and in the middle of the parking lot check around the tree to find this spaceship part.
Spaceship Part #26
¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯
Another pretty easy one. Do you know where the Galileo Observatory is? It's in one of the main story missions. It is slightly northwest of Lake Vinewood. Head up there and check the left side of the building for some stairs leading up (jump over the barricade). Search the roof across from the binoculars for the part.
Spaceship Part #27
¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯
This one is easy as well. Head to Banham Canyon, where the Banham Canyon Drive meets the Buen Vino Drive intersection. Head down the road west a bit until you see a driveway (covered driveway) on the right. Ram through the doors and go to the house. Get out now and head around the right of the house to see this part by a nearby telescope.
Spaceship Part #28
¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯
This one may have a little more vague directions then the rest. You need to find a cove on the east coast of the island. It is east of the "Ron Alternates Wind Farm", north of the "Palmer-Taylor Power Starton" and south of the "Davis Quartz". Once you find it it is very obvious, as there are some jet ski's and a boat nearby.
The spaceship part is underwater in the cove. Dive down and search along the right wall sea floor to find it.
Spaceship Part #29
¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯
This one is a bit tougher due to it being by an un-named road. Find Mt. Haan Road, north of the VINEWOOD sign. On the northeast side of that road is a dirt road heading north and making an "Z" sign. Head down that road and after the third swich-back (turn in the road), go check out the slope of the hill north of the road to find this part.
Spaceship Part #30
¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯
This one isn't too hard. It is located in Marlowe Vineyards, which is in the north area of Tongva Valley. First locate Buen Vino Road. You can find all of the vineyards on the road to the east. There is one section of the road that dips to the shout and makes a boot shape. Get to the eastern side of the boot and look for a small dirt road leading up to the vineyard. Go up it and head up the hill. Look to the right for a row of grapes with two baskets sitting nearby. Head down two more rows then go inspect the rows to find the part.
I like how you explained where the parts are and just incase added a video well done