At my current job I have both computers on my desk. In most fleet squadrons they have a couple computers in a room/vault somewhere. Not easy, but not impossible either.
As a RAG instructor I had to try to get SIPR in the squadron spaces. LOTS and LOTS of BS paperwork, abject "No Way" from the Wing computer homo, and general red-a$$ from everybody in The Peoples Republic of Cherry Point. Depending on where you're at, you may have it on your desk, in your squadron somehwere, or maybe in another building (painful).
Just as a bit of advice, whenever you send somebody a SIPR e-mail, make sure you follow up with NIPR or a phone call. I keep forgetting that some people don't have ready access and won't know they have an inbound message rifht away unless you give them a heads up.