I'm sure any 12 volt American flasher should work. Any parts store would have one. A Wagner 552 flasher comes to mind. If you have to make a couple of pigtails its no big deal.
Just a thought, why not purchase a cheap revolving amber beacon? I now use them as I feel they are more noticable to others on the road. (plus I'm told they made it law here in NY for tractors to have a revolving light when on the road). I think its good as I've seen many a tractor traveling at dusk and they couldn't be seen period. I use my beacon for safety as people drive like wackos now a days. The cheap beacons are only like 20 bucks. I think the expense is well justified.