once you get outside the light duty reels with molded connectors or a light on the end, they start getting expensive.
Cox makes a reel which will accomodate 30 feet of 12/3 SJO and is rated at 300 volts. The part number is PC10L-3012 (you can get it from them with cord on it as well, but if you do confirm the cord is SJOW as their spec sheet says SJO but I am pretty sure all SJO is really SJOW these days) Cost should be in the $200s.
Both McMaster and Grainger carry cord reels that don't have pre-attached plugs, which are rated for up to 600VAC. They are expensive and you need to add on the cost of suitable end caps.
Would a 'self retracting' cord be suitable? This is an SO cord that has been molded to take a spiral form, so it can stretch out to 4-5x its retracted length. This eliminates the need for slip rings, reel, bearings, etc, and greatly reduces the cost...but only for limited lengths. http://www.philatron.com/NewCatalog/TAB7/Power_Coil_Cables_18-10Gauge.html