Ebay, Harbor Freight(It's Texas, I KNOW you have one), Wal-Mart.

I got mine off of EBay and it weighs up to 35lbs, and weighs in 5g increments. Cost less than $25.

If your boa gets to be 50lbs, which I doubt he/she will unless you're feeding her huge meals, then get one of those hanging fish scales and hang her on it in a large pillowcase. Or better yet, weigh her in your arms on a human scale, weigh yourself and subtract the difference... Not too hard.