When it comes to P-90s, I always recommend either going big or going small, and ignoring the middle ground.
To that end, the best of the boutique hum-sized P-90s I've found comes from The Creamery. There's a few different options for their Hum-90 pickup. They're wound with 43 AWG wire to either 9k or a little over 8k (i.e. bridge & neck, but you can use either in either position) and you have a choice of alnico 2, 3, 4 or 5 magnets. I use the alnico 4 for the neck to get an even balance closer to a full-size, normal P-90, and an alnico 2 for the bridge to get a fatter P-90 tone, like an overwound PAF with more clarity through the lower frequencies. At £80 a pop they're not actually the most expensive of the boutique winders, but they still cost more than common brands, and if you have to pay large shipping fees on top of that, it can be pretty pricey for what is essentially just two magnets and a length of wire.
If you're not keen to pay out for that, I recommend you skip right down to the 'budget' lines. P-90s are such a simple construction, and there's no coil balancing to worry about, so it's very easy for cheaper brands to put together a very nice pickup, compared to how often they fail at making a nice humbucker.
Irongear and GFS do make a few different P-90s in humbucker housings. For my money, the Irongear Alchemist 90 is better than any of the GFS ones I've tried, but that's mostly a matter of taste. The Irongear P-90 is underwound so it is a little clearer, while all the GFS ones are wound hotter to various degrees, so you can pick pretty much any balance of clarity and power that you like.
The king of the cheaper, hum-sized P-90 is the Tonerider Rebel 90. The neck is an almost exact clone of the SD Phat Cat—only it saves you a few coins—and the bridge is wound just hot enough to give you that P-90-meets-PAF feel that is what hum-sized P-90s are really best for. If you think the price of the boutique Creamery pickups is too high, the Rebel 90s should be your #2 choice. The sensitivity is unmatched until you get to the boutique pickups.