It doesn't sound like you know where they're buried, so I can give you that & the dates at least. Referring to this cemetery list compiled by H. Lyle Hinz of the Brown County Genealogical Society, your Joseph & Sarah are buried in the Kimberlin Cemetery in Washington Township, Brown County, Ks.
It's located 1.5 miles East of Everest, Ks., then 2 miles North. The cemetery is very small, consisting of just over 40 graves. I'll include the entire list of ARMSTRONG buried in that cemetery, since some of the others are probably related.

Emma E. 16 AUG 1887 - 5 SEP 1910
J.L. 4 JUL 1821 - 4 OCT 1901
Sarah C. 29 JAN 1819 - 5 AUG 1874
Robert P. 7 DEC 1884 - 6 DEC 1886
Earnest R. 20 JUL 1890 - 11 JAN 1891
William O. 1860 - 1928
Mary J. 1856 - 1935
Leroy O. 1918 - 19__
Elizabeth 1882 - 1966
Albert O. 1882 - 1937
Louisa E.(no birth date)- 31 JAN 1885, "29y 5m 19d, wife of S."
William T. (no b. date) - 26 OCT 1885, "6y 10m 25d, son of S.& L.E."