Utila has a decent supply of basic stuff. You'll find a small but relatively standard range of shampoos, etc in our main supermarket (Bush's). If you need or want anything specific or even a little harder than normal to get (i.e. not on the shelf of every store!) then you should bring that.
As for usual stuff there is no problems, we have good supply from the mainland. My girlfriend and me haven't need to go to the mainland for anything for the last 9 months. As for clothes, the laundries here generally will have you dry clothes ready for you in less than 24 hours.
Things you should focus on would be batteries for cameras, computers, strobes, etc - even if you can buy them here they won't be as good or last as long as those you bring with you. Any spare you may need for your dive gear especially if you have gear that is not Mares or ScubaPro. If you have your own dive light(s) then bring your spare bulb(s) if it isn't LED.
A back up timer or compute is always a sensible thing to bring with you as well if you can. You can pick up an adequate timer for a 1 month trip under $50 pretty much anywhere.
Finally, you will almost certainly need a lot less than you think. A few years ago I traveled for a year on a round the world dive trip. I packed a 40lb bag and didn't use half of it. I could have comfortably traveled for a year with 20 to 30lbs of clothes, cameras, books, toiletries, etc.
-rob