I've tested a variety of different solutions for protection in sparring. I have found nothing better suited to the task than the BTS High Gear system. This offers a similar profile to boxing/sparring headgear with wrap-around eye protection, full face cover, full peripheral vision, light weight...
The problems with fencing masks is they tend to have an unwieldiness about them; the metal face mask pulls the helmet forward, and it's often held in place by a piece of padded wire, and occasionally an elastic strap. This makes it slide about quite a lot, and it doesn't offer too much in the way of back of the head protection (I have no idea what sort of training you're doing that has rules for knife fighting, so I have no idea what would need to be covered).
The High Gear helmet is quite similar to the boxing helmet with goggles, but with full head protection. The down side, of course, is going to be the price – $370 for the helmet alone; a throat guard is available for an additional $75.
Less impressive options are any of the number of various forms of martial arts face masks/shields which offer inferior protection, especially in the region of the eyes. Alternately, you might look at how various types of face shields from other fields might suit your needs, since these will likely be far cheaper, but also less likely to offer the protection required (good stress testing is a must before use).