class X supression caps

A lot of class X1 supression caps seem to be self-destructing in vintage equipment these days. I've got one in my NCR needing replacement, plus we have an HP 3000, two Apple Lisas and an Apple /// all needing them too... (they don't half smoke when they let go too :) Looking around, it seems that everywhere these days sells class X2 caps, and Y2(X1) caps. What's the modern-day replacement for a dead X1 cap? X2, Y2(X1), or something else entirely? I'm sure I've seen Y2 caps in old equipment as being a different thing to X1, which is why I'm getting confused if they're now classed as the same! (Anyone have a source for 0.68uF X1 caps in the UK? Seems to be a very uncommon value for supression caps - everyone does 0.47uF and then 1uF) ta Jules