I don't know what your cost target is, but here are some suggestions.

Aluminum lamp posts are available commercially. Search on line for manufacturer and dealers.

Aluminum and stainless steel pipe can be found in a wide range of sizes. It is usually stocked in 20 foot lengths. Look under metal suppliers in the Yellow Pages. You can sometimes find usable pieces at scrap metal dealers for a whole lot less money.

A flagpole could be modified. They are available in aluminum and fiberglass. Look on line for suppliers and dealers.

Pressure treated 4x4s and 6x6s are available at just about any lumber dealer in lengths up to at least 16 feet. Decorative caps are available.

If it were me, I would probably go with the latter.

My lampost/lamp is a cast metal unit. The light is about 6 ft high or so. May I suggest you make a concrete pad to fasten the unit to? I cut the bottom off a 5 gal paint bucket, inverted it, and filled it with concrete, then cut a slice in the side to remove.