You'd need an AR or someone with one. You can try posting on the Trade boards and ask if anyone will be willing to find yours out for you. All you'd have to do is trade them a Pokemon (any one) and they can do it for you.
Pokemon Platinum FC: 1891 7076 4490 Standard Rules
Prolly too late but you can use Red Gyarados to find your SID if it's untouched or if you somehow remember EVERY single pokemon you fought.....and I think on RNG Reporter there is a calculator to find your SID with Shinies just calculate it with Red Gyarados or if you have a shiny you can use that. You should also calculate the Gyarados multiple times if you didnt catch it yet so it can accurately calculate your SID
=/ B4 I used RNG it took my 3 months to get a Shiny Sudowoodo....
Pandas are the least racist creatures on the planet because they are black, white, and Asian.
more specifically, it requires knowing the IV's for any caught shiny that is not the LoR Gyarados, or a list of IV's for about 7 LoR Gyarados (which are obtained by fighting it, catching it, figuring out the IV's, resetting, and repeating).
So basically, if you've caught any shiny besides LoR Gyarados, we can find your SID and figure out how exactly to get you that Rayquaza without having to spend weeks SRing. Btw, a friend of mine spent 6 weeks SRing to get his shiny Rayquaza. Just saying.