We just recently bought a new inverter. After selecting the model we wanted, we went to the manufacturer's webpage and found their list of dealers. We then sent each dealer an e-mail asking for a quote and availability. Most of the dealers of the one we went with were marine shops.
We ended up saving about $700 off the 'store' prices by putting it out for bid like this.
If the inverter you're looking for is more commonly available (ours is a unique one that is mostly used in Europe and/or on boats), you might try looking on Craigslist, eBay, etc. as well.
Best wishes,
- Cherie