I recently ordered an Orange Thunderverb and it came from the UK. It's voltage selector switch on the back is for 220/230V. Obviously that doesn't work with 110/120V in North America. What're the options; UK kettle lead and a step up transformer?

Thanks for the suggestions
Yup, that's a good plan. Get a step up transformer rated higher than the amp's power rating, and check the back of the amp for the actual power consumption (not the output power). 

For example, a "100 Watt" DSL actually more uses more like 400 watts, it'll say on the back next to the power socket. From there, give yourself a healthy buffer when choosing a transformer (150% or 200% of the rating, or more if you want). I'd suggest running the transformer after a power conditioner to make sure the incoming voltage is stable and correct before you start multiplying it.