1) Pay for advertisement.
You can pay for your entire page to be advertised, or a specific post or picture. This can get quite expensive, but it's a solid investment if you advertise to the right people.
2) Use your personal profile to advertise you page.
This way only reaches your friends, but it doesn't cost anything, and you can ask your friends to share your page, as well.
3) Utilize "buying and selling" Facebook groups.
There are a lot of "yardsale" and "small business" groups on Facebook that are meant for a specific area, such as a city or county. These groups are great for reaching locals that could potentially be looking for what you're selling. The only drawback, however, is the fact that most of these groups are for "yardsale" items. The people that are in these groups are looking for a deal, and custom cakes were never meant for deal-making, so proceed with caution.
All of that being said, don't go on that sort of a group and do this:
I went to my local Facebook "buying and selling" group, typed in "cake" and got that^^. That's extremely annoying, clogs up the group posts, and reeks of desperation. Do that, and everyone instantly doesn't want whatever you're selling. Keep your posts at a minimum. If people want it, they'll come to you.