Hey Sam, see my other post about this, you can buy a 110v transformer from MJMP (http://www.marshalljmpmodshop.net/miscellaneous_parts_and_replacement_tubes.htm). But meantime you could use a step up transformer (117v to 220v) to run it, need to watch out for the power plug configuration, i.e. the step up transformer needs the correct outlet for whatever power plug your 220v Classic has. There is no "adapter" to change voltages, you need a step up transformer which takes 117v (US voltage) and converts it to 220v. Here in Australia we/I use step down transformers 240v to 117v to run US or Japanese units and these have US style power outlets on them.
Depending where (which country) this Classic you bought is coming from, it will have a plug for that country, they are all different but mostly (for ADA gear) it would be a UK plug, a Euro plug or Australian plug.
Power plug adapters (like the ones you can get for overseas travel) work for gadgets that can work off multiple voltages (e.g. laptop power supplies, electric shavers etc.) but have different plugs. ADA gear was made before multiple voltage capability was a thing.