Traxxas used to make various springs for the big bores years ago when they came on the SRT and TCP racing vehicles, but they have been long since discontinued and you aren't going to find them anywhere - the stock red springs are the only ones still made (2.5 rate). Take a look at Losi springs, as they seem to be the best fit on these shocks and experiment with different lengths and spring rates. There are other springs available from Traxxas (progressive and older ultra shock springs), but they fit a bit looser around the shock body.