I searched the forum and I might have missed a sponsor, but what recommendations do you have to source boat registration decals?

Try most any sign shop. That's where I get mine.

Academy Sports, Bass Pro, Walmart probably has them.

So, here's how it works. More than likely your local state boat registration is going to say you need 3" tall BLOCK letters that contrast with the hull. These you can buy most anywhere. Cabelas, Bass Pro, Home Depot, and so on. I had some custom made to try closely match the font of my boat. I did them in white rather than silver just in case. Knock on wood, I haven't had any problems with IL or WI law enforcement. Had them made by iguanagrafix

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The registration decal, the state mails to me, the letters I have made.

Most sign shops the do vinyl lettering can custom cut you a set for not a whole lot more than you pay for those you buy off the shelf.  In GA, they just say they have to be contrasting colors, not ugly contrasting colors, and that does not mean they have to be black on white, so I've been having mine custom cut just about ever since they started doing it.

As long as you don't get too radical with the style, and keep it a very legible style, I don't think many states will say anything about using something better looking than just square block letters.  I've been doing it for about 15 years, been checked a ton of times by GA, SC, TX, AL, and FL game wardens and no one has ever said a word about mine.  Actually, the ones on my Javelin are not so contrasting, they are a metallic that almost blend into the stripe in one spot, and I figured they would make me change them, but it's been at least eight years and checked a bunch of times.

I had mine custom made to match my color boat as well. It is not block letters and similar to slonezp's text. I have had my boat with this text for 7 years and never had a problem with it. As long as the numbers and letters are clearly discernible, you shouldn't have a problem with it.

I bought mine at Menards but Home Depot has them as well. They are the 3 inch block size with adhesive peel off backing available in white or black.

Call a sign shop that does vinyl letters.  You can get the font and color you want and you'll ensure they are on straight.  Probably cost you about $20 and they'll have them ready in a couple of hours.