I would recommend trying an agency out first before you throw your cards in on a big job... I've had many no show/no calls this year and it's super frustrating. If the temp manager is good, he will find you guys with similar skills (wildland fire, forest service, etc). Once you've worked with a couple decent guys, you can bring them in for the bigger jobs and get sht done.
Also, you can often reduce overall hourly cost by paying them 'unanticipated bonuses'. This keeps the rate at like 16%, compared to all hours run at 50% on the dollar (in my case at least).