Bryan, in January there is going to be a European auction selling the world's best giant veg seeds including several from world records. This will include giant cucumbers, radish, amaranth, tomato, marrow, beetroot, carrot (heavy/long), parnsips (heavy/long), sunflowers, etc.
You could also join the EGVGA and you will get a lot of giant veg seeds for free.
Check out: http://egvga.webs.com/members.htm
and the homepage: http://egvga.webs.com/
It's the cheapest way to get seeds. There will be all kinds of giant veg in there. For sure giant parnsip and giant beetroot and depending on how much members send in, likely either/and/or marrow, tomato, squash, long gourd, etc.