Be aware that there are caveats in this approach. First, a /24 or larger range is compulsory for multi-provider BGP, which is, itself, compulsory for dynamic advertisement in the way you want. Second, broker ranges are only suitable for your use case if they are allocations rather than reassignments. Third, even when a broker has reassignable ranges, they need to then use detailed reassignment rather than simple reassignment for your new block. Otherwise, you will be unable to announce prefixes to providers requiring proof of ownership. Please refer to https://www.arin.net/resources/request/reassignments.html for detailed information on the SWIP process.

In every recent case I've seen (over the last six years), no broker has had ranges larger than /24 for reassignment. Of course, you can do with just a /24, but the justification process has also grown somewhat more stringent, as a pure traffic engineering justification ("We need BGP; here is why.") is no longer enough, in my experience.