Quite a few people expressed envy about having a budget for models in the comments section of my invoice post. Yes, most large Publishers will pay for a professional model. But even if you aren't doing a professional commission, there is no reason you shouldn't hire a model.
Having a good model will greatly improve the quality of your reference, and thusly your painting. Quite often, I have had models come up with poses and ideas that I never would have thought up, and it ends up making the painting for me. Yeah, you could pose for yourself, or have your buddy or girlfriend pose... But having a really beautiful person in front your lens is an incredible inspiration all it's own. Plus, you don't have to spend all that time guessing what it would look like if you actually stuck to that sit-up regiment like you promised you would!
One of my favorite resources for models is a website called Model Mayhem. Unlike a lot of other modeling websites, MM is totally free to join for both models and photographers. The result is a really large pool of talent to pick from.
No matter where you live, there is likely someone who would like to pose for you. Many of the models, being amateurs themselves, are even willing to pose for free. These people will be listed as willing to work for TFP... meaning: Trade For Prints. This means they are willing pose for a reasonable amount of time, and in exchange you provide them with portfolio worthy photos. It's a win-win for all.
Obviously, the pickings will be slimmer the further away from a big city you live. But don't underestimate the talent available. I live in a very small town of just 5000 people in Connecticut, and below are some of the amazing models I have had the great pleasure to work with.
As you find better models, their rates will likely be higher than those just starting out. Typically, a professional model in NYC gets paid about $150/hour for a commercial illustration shoot (If they are doing an advertising campaign, and the final product is a photo where their likeness will be obviously recognizable, it is a totally different thing and I believe they get paid differently). Here in CT, I pay my models anywhere from $50/hour to $100/hour depending on their skill level and how difficult the shoot is. However, if both the artist and the model are amateurs, you could likely spend $25/hour and still find a great model.
Doing a professional quality painting is a major time investment, and you are doing yourself a great injustice if you are not willing to spend a little on good reference. In the end, the time (and frustration) you save yourself by having good reference is well worth the time expended acquiring it.