Does anyone have any experience in getting from Suvarnabhumi airport to Chantaburi?

What did you find the best way?

I'd prefer a private van or even a taxi, but would rather have it sorted before I get to Bangkok.

If anyone has a recommendation, or even better a contact number, I'd appreciate it.

The last time I did it (about a month ago), I just went outside the terminal at the arrivals level and talked to a few of the many drivers looking for business. They'll find you if you're not sure who to approach. I found a taxi to Chanthaburi for 2,000 baht. I think it's hard to find a cheaper price than that with current fuel prices. This was about 11 in the morning. If you arrive late in the day or in the evening it may be more difficult since the driver will want to get back to Bangkok at a reasonable hour. Enjoy your trip.