Amazing how long spar has been around and still the finish of choice for so many (especially mariners). Like they say, "When something works..."
There are so many urethanes out there now, and even the waterborne products are beginning to develop durability on exterior applications, it's almost impossible to recommend one.
Defthane is always a good choice, maintains a lot of the amber appearance of spar, and definitely dries faster, but there's also MinWax urethane products, including Helmsman Spar Varnish, which you should be able to find at the corner hardware store. Then there is Varathane, one of my personal favorites; comes in a variety of formulas, including exterior-specific. I have an oak table in my study that was "Varathaned" almost 20 years ago and has gone through 4 kids without a sctratch...
Luck with what you choose, if you're attached to using spar I'm sure you can still find it...