I asked the same question in about April of last year and I think both Freddd and Rich said that it shouldn't matter - that methylation protocols are a very long term process and whatever B12 you're currently taking won't matter. The test doesn't measure B12 levels, just glutathione and other end-products of a functioning methylation cycle.
Hope that helps. I tried to find the thread where I asked that question, but it didn't come up.
Funny you ask because I had also posted a thread seeking advice about this (and again, I can't find it). Sushi was particularly helpful. But the answer is: my local hospital. It's very difficult to get commercial labs to do it (like Quest) as most have a policy where they won't draw blood and hand it over to you OR mail it someplace other than their own labs. But most hospitals have labs that will draw blood and ship it off for you.
Someone mentioned that you want to get the blood drawn as close as possible to the lab's FedEx pickup time, so the blood is not sitting around. You can call and find out when their pick up time is.
The actual form that comes with the test kit says you should be off all supplements for 2 days before the test. I had to reschedule my tests last week because the doctors office had forgotten to tell me this.