Most OMGSRs are worth different amounts in pets, it's not going to be one straight answer. The way that pet values work is that an OMGSR would be worth thousands of uncommon pets, which most people don't have. When you trade bulk pets for a single or a few pets, there's what is called "quantity value". Although the bulk pets don't actually add up to the valuable pet(s), they're worth a bit more due to how many there are.
Most players who own OMGSRs probably aren't going to trade them down for even thousands of uncommons without a major reason (collecting, hoarding, etc), unfortunately. Probably the best way to find someone who's willing is have a note in your trade thread or trading rules about it so they can easily see that you're looking to trade your uncommons for an OMGSR.
I highly suggest taking a look at
Horror's thread to not only see how to trade but also what OMGSRs go for.
Here are some ways to get higher rarity pets, some even include VRs and OMGSRs:
Art/Character/Coding Shop - There are a ton of people on CS who pay either C$ or CS pets for art of their characters, creation of custom characters (either through your own art or bases) or coding (usually for trading rules or signatures). If you find yourself able to do either of those things, that could be a quick way to get some high quality pets.
Pound / Lost & Found - The Pound, as you might know, rereleases pets of all kinds of rarity a couple of times a day. Some can even be VR or OMGSR! These pets go very quickly, so you have to be at the opening the second it opens or you might miss out! The Lost & Found is similar, but for items! Some items are very valuable and worth VRs and OMGSRs in pets.
Events - Especially the Easter, Summer and Halloween events, you will get higher value pets. Most of the token trade-in pets for those three events turn into rares. Most of the smaller event pets turn into uncommons typically.
Trading - This one is obvious, but using the pets you have and trading up for the ones you want is a good way to get more pets!
Monthly Adoptions - Adopt all of the pets every month, no matter what. These won't turn rare typically but a lot of the litters will have uncommons in them! They make for good fodder in the long run.
December 18th - If I were to suggest anything in this thread, it would be december 18th. I'm not exactly sure what happens or what it looks like, as I haven't experienced it yet, but from what I hear, you get three rare, VR, or OMGSR pets on December 18th. Be careful that you adopt them before CS rollover, or they'll be gone.
If anyone has any other suggestions I missed, feel free to add on!