I had a couple of seasons riding cycle speedway in the early nineties. It's great fun and teaches you just how much physical contact you can get away with on a bike witout crashing. On a track that small there's no such thing as a clean pass!
Back then nearly every cycle speedway bike I saw was an "Ace Ultra" made here in the Midlands by a guy called Mike Baugh. He has relocated to a different town but still has a shop http://www.aceultracycles.co.uk/General/about-us.html
although I don't know if he still has any current links to the sport. If you're looking for retro then this would be the way to go although it seems (as Al pointed out) that Archie Wilkinson bikes are at the forefront of cycle speedway now.
I don't know what's current but I found this website which has a bit of info, especially in the "pits" section http://18.104.22.168/CycleSpeedwayTrack/index.html
. I have seen the odd bike come up on Ebay but never watched one to see what sort of money you'd be talking.
Below is the only thing I have left from my dabble in cycle speedway. I rode for Sandwell, mainly the "B" team, but was part of the under 21 team that made the national final. Sounds quite impressive but we were absolutely thrashed by the winning team!