You do not want to have any splices underground, i.e. using two 100' pieces of tubing plus another 15' isn't recommended because of the shrink/expansion of the underground tubing tends to pull connections apart.
1" Black plastic poly can be obtained in 300' lengths, which would be ideal for you. I ordered mine from Home Depot, but any home improvement store or plumbing supply house should be able to get it for you.
NOTE: I have an underground splice at the 300' mark - at 600', I have an above ground manifold assembly to allow switching between deep water (Summer) and shallow water (Winter ice breakup) aeration. Fortunately, that underground splice, consisting of a plastic coupler, 4 stainless steel cable clamps, and lots of electrical tape (which IME is pretty darn water proof) covering all of it, has lasted for 19 years. OTOH I had to replace a brass 1" ball valve at the manifold this Spring after it split due to being struck by a falling tree (Thank you, Emerald Ash Borer!).
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