We made a custom worm gearbox for our robot's 'Shoulder' last year, and considering that it was done mid-season, it worked really well.
The Gearbox had a total 480:1 Reduction and used a COTS 16DP Worm and Worm Wheel From Boston Gear. (Purchased Through Amazon Supply.) The initial stage consisted of a 15T 32DP Pinion Driven by an RS-550, to a 60T 32DP Gear From AndyMark. From there, the 60T Gear drive a Single Start Steel Worm, which in turn drove a 40T Brass Worm Wheel. There was an additional 20DP Spur Stage after the Worm Wheel that allowed us to Vary the boxes overall reduction should we have ever needed to.
In the case of AndyMark's Worm Boxes (Both the Worm Box and Raw Box), I believe they both use multiple start Worm Gears which will have less back drive resistance than a single start worm. Depending on how exactly they're integrated into specific setup, you may be able to get away the amount of resistance provided, or you may not. I wouldn't be surprised if there's some other COTS worm gear pairing that is the same Pitch and approximate overall dimensions that swaps into AM's boxes without too much work.
If you decide to go down the Custom Worm Gear path, here's a worthwhile document from Boston Gear:
http://www.bostongear.com/litportal/...BG_pg73-86.pdf
I found that most of Boston's common sizes (12DP, 16DP, 24DP) were readily available through a variety of online resources, one of those being Amazon Supply/Amazon. If I remember correctly, the gears that we purchased were a 'prime' item, so they shipped in 2 days for free.