Calculate the frequency corresponding to a given current or power attenuation in an LRC circuit
I'm not totally sure what you are trying to do, but you can just plot the amplitude of I vs. frequency.
As an example (R = 20Ω, L =  26.5mH, C = 10µF, and E = 1V peak sinusoid) below is a plot of steady-state current amplitude vs. frequency using a simulation tool (ATPDraw in this case).

EDIT:  You can curve fit to get equation for frequency.  I just did using Curve Expert:
name:   Polynomial Regression (degree=7)
equation:   a + bx + cx^2 + dx^3 + ex^4 + fx^5 + gx^6 + h*x^7
Parameters:
a = -2.99513035735665E-05
b = 6.72435685497032E-05
c = -1.48237074557974E-07
d = 1.92511125408853E-09
e = -1.21174827540529E-11
f = 3.11454508578765E-14
g = -3.63866095853704E-17
h = 1.61817957432187E-20

Beyond that, i'm not sure how to help you.