Microsoft does not sell 2003 versions any longer. Your only option would be an after market sale from somplace like ebay and that runs the risk of licensing issues.
I have not tried the 2007 runtime but my client is using A2007 to run an .mde created by A2003 so I'm pretty sure that the runtime will do it also. That would mean that you don't even need A2007 on your development PC. You can work entirely in A2003 and as long as you create menus and/or toolbars, you should be ok with the runtime. You can also test the effect of using the runtime by opening your application with the \runtime switch. This allows the retail Access to simulate the runtime Access.
Also, since you created your app with A2003 and are using custom menus, the ribbon is hidden and only your menu/toolbar show which saves a lot of precious vertical space for those using laptops.