Here are some resources for finding historical interest rates.  I searched very long and hard to find this kind of data.  Hope it helps.  While you are here, feel free to browse around if you are in need of some more interest income, or less interest expense! 

Historical Interest Rates - Mortgage

I have the following four data sets in an excel spreadsheet side by side which you are welcome to request absolutely free below:

    Market yield on U.S. Treasury securities at 1-year constant maturity
    Average majority prime rate charged by banks on short-term loans to business
    Market yield on U.S. Treasury securities at 10-year constant maturity
    CONTRACT RATE ON 30-YEAR, FIXED-RATE CONVENTIONAL HOME MORTGAGE COMMITMENTS

1974M1	96.57
1974M2	96.22
1974M3	93.98
1974M4	90.31
1974M5	87.28
1974M6	86
1974M7	79.31
1974M8	72.15
1974M9	63.54
1974M10	73.9
1974M11	69.97
1974M12	68.56
1975M1	76.98
1975M2	81.59
1975M3	83.36
1975M4	87.3
1975M5	91.15
1975M6	95.19
1975M7	88.75
1975M8	86.88
1975M9	83.87
1975M10	89.04
1975M11	91.24
1975M12	90.19
1976M1	100.86
1976M2	99.71
1976M3	102.77
1976M4	101.64
1976M5	100.18
1976M6	104.28
1976M7	103.44
1976M8	102.91
1976M9	105.24
1976M10	102.9
1976M11	102.1
1976M12	107.46
1977M1	102.03
1977M2	99.82
1977M3	98.42
1977M4	98.44
1977M5	96.12
1977M6	100.48
1977M7	98.85
1977M8	96.77
1977M9	96.53
1977M10	92.34
1977M11	94.83
1977M12	95.1
1978M1	89.25
1978M2	87.04
1978M3	89.21
1978M4	96.83
1978M5	97.29
1978M6	95.53
1978M7	100.68
1978M8	103.29
1978M9	102.54
1978M10	93.15
1978M11	94.7
1978M12	96.11
1979M1	99.93
1979M2	96.28
1979M3	101.59
1979M4	101.76
1979M5	99.08
1979M6	102.91
1979M7	103.81
1979M8	109.32
1979M9	109.32
1979M10	101.82
1979M11	106.16
1979M12	107.94
1980M1	114.16
1980M2	113.66
1980M3	102.09
1980M4	106.29
1980M5	111.24
1980M6	114.24
1980M7	121.67
1980M8	122.38
1980M9	125.46
1980M10	127.47
1980M11	140.52
1980M12	135.76
1981M1	129.55
1981M2	131.27
1981M3	136
1981M4	132.81
1981M5	132.59
1981M6	131.21
1981M7	130.92
1981M8	122.79
1981M9	116.18
1981M10	121.89
1981M11	126.35
1981M12	122.55
1982M1	120.4
1982M2	113.11
1982M3	111.96
1982M4	116.44
1982M5	111.88
1982M6	109.61
1982M7	107.09
1982M8	119.51
1982M9	120.42
1982M10	133.71
1982M11	138.54
1982M12	140.64
1983M1	145.3
1983M2	148.06
1983M3	152.96
1983M4	164.42
1983M5	162.39
1983M6	168.11
1983M7	162.56
1983M8	164.4
1983M9	166.07
1983M10	163.55
1983M11	166.4
1983M12	164.93
1984M1	163.41
1984M2	157.06
1984M3	159.18
1984M4	160.05
1984M5	150.55
1984M6	153.18
1984M7	150.66
1984M8	166.68
1984M9	166.1
1984M10	166.09
1984M11	163.58
1984M12	167.24
1985M1	179.63
1985M2	181.18
1985M3	180.66
1985M4	179.83
1985M5	189.55
1985M6	191.85
1985M7	190.92
1985M8	188.63
1985M9	182.08
1985M10	189.82
1985M11	202.17
1985M12	211.28
1986M1	211.78
1986M2	226.92
1986M3	238.9
1986M4	235.52
1986M5	247.35
1986M6	250.84
1986M7	236.12
1986M8	252.93
1986M9	231.32
1986M10	243.98
1986M11	249.22
1986M12	242.17
1987M1	274.08
1987M2	284.2
1987M3	291.7
1987M4	288.36
1987M5	290.1
1987M6	304
1987M7	318.66
1987M8	329.8
1987M9	321.83
1987M10	251.79
1987M11	230.3
1987M12	247.08
1988M1	257.07
1988M2	267.82
1988M3	258.89
1988M4	261.33
1988M5	262.16
1988M6	273.5
1988M7	272.02
1988M8	261.52
1988M9	271.91
1988M10	278.97
1988M11	273.7
1988M12	277.72
1989M1	297.47
1989M2	288.86
1989M3	294.87
1989M4	309.64
1989M5	320.52
1989M6	317.98
1989M7	346.08
1989M8	351.45
1989M9	349.15
1989M10	340.36
1989M11	345.99
1989M12	353.4
1990M1	329.08
1990M2	331.89
1990M3	339.94
1990M4	330.8
1990M5	361.23
1990M6	358.02
1990M7	356.15
1990M8	322.56
1990M9	306.05
1990M10	304
1990M11	322.22
1990M12	330.22
1991M1	343.92
1991M2	367.07
1991M3	375.22
1991M4	375.35
1991M5	389.83
1991M6	371.16
1991M7	387.81
1991M8	395.43
1991M9	387.86
1991M10	392.46
1991M11	375.22
1991M12	417.09
1992M1	408.79
1992M2	412.7
1992M3	403.69
1992M4	414.95
1992M5	415.35
1992M6	408.14
1992M7	424.21
1992M8	414.03
1992M9	417.8
1992M10	418.68
1992M11	431.35
1992M12	435.71
1993M1	438.78
1993M2	443.38
1993M3	451.67
1993M4	440.19
1993M5	450.19
1993M6	450.53
1993M7	448.13
1993M8	463.56
1993M9	458.93
1993M10	467.83
1993M11	461.79
1993M12	466.45
1994M1	481.61
1994M2	467.14
1994M3	445.77
1994M4	450.91
1994M5	456.5
1994M6	444.27
1994M7	458.26
1994M8	475.49
1994M9	462.69
1994M10	472.35
1994M11	453.69
1994M12	459.27
1995M1	470.42
1995M2	487.39
1995M3	500.71
1995M4	514.71
1995M5	533.4
1995M6	544.75
1995M7	562.06
1995M8	561.88
1995M9	584.41
1995M10	581.5
1995M11	605.37
1995M12	615.93
1996M1	636.02
1996M2	640.43
1996M3	645.5
1996M4	654.17
1996M5	669.12
1996M6	670.63
1996M7	639.95
1996M8	651.99
1996M9	687.31
1996M10	705.27
1996M11