Assuming its legal in your state (and I'm fairly certain it is in OK), you merely have to find a supressor you want, and fill out a BATF form 4. You'll also need pictures, fingerprints and a check to the Treasury Dept for $200. The hard part is getting the local CLEO (Cheif Law Enforcement Officer) signoff. You have to go to the sheriff, cheif of police, or a judge in the county of your residence and get them to sign off on the form. If you can get the local CLEO signoff, its a relatively easy process, it just takes a while to actually get the supressor in your hands.
Now the mechanics of how to find and buy one. If you have a local class 3 dealer or there is an individual in your state who has a supressor you want to buy, you do it direct on the form 4. If you want to buy a supressor from an individual or a dealer in another state, you have to run it through a dealer in your state first. The individual in another state will have to do a form 4 to the dealer in your state (complete with the $200 tax). From your dealer you will do another form 4 to get it into your hands. If its coming from another dealer or a manufacturer in another state, they'll transfer it tax free to the dealer in your state on a form 3 - then you'll do your form 4. Form 3's generall run a few weeks, while form 4's run a couple months (so if you buy from an individual in another state, you might not get your supressor for 6 months or so).
GemTech is the Rolls Royce of supressors. They're expensive, but they're damn good, and GemTech sets the standard for excellence in customer service. You have a problem with one of their products, they will make it good; no fuss, no muss.
There's a lot of good small manufacturers too. About the best place to find something is
subguns.biggerhammer.net/nfaad.cgi