Unless you luck out and someone has one to share, you will either have to learn FileMaker and completely build one yourself, or hire someone to build it for you. I feel your pain. The complete software package I needed for my small business was over 20K (HVAC), so I decided to build one myself. With the help of these forums, and especially Phil, it works great.
Some people try to use/modify the starter solutions.
I would recommend really learning FM. What really helped me was the FileMaker Training Series (for your version), and The Missing Manual (your version). What really helped the most was the video training series here:
About 50 hours of videos, and as you learn you go step by step and build an actual Contact/Invoicing Solution. By the time you worked thru that, you could easily add the inventory part.
And of course come back and ask questions. Most of the help people receive is for specific questions. More of the ...how do I build a database...tend to receive less help.
You could also google FileMaker templates, or free FileMaker templates.