First, check power input details located at the bottom of your laptop (No Model specified, and HP makes a lot of different Laptops).
For a small profile, perhaps the Cooler Master RP-095-D19A-A1 SNA 95 Slim Notebook Adapter (about $60):
http://www.newegg.com/Product/Product.a ... -_-Product
This adapter has a reasonably small form factor, yet is about 100W. Reviews are pleased with how it charges batterys.
You can probably get by with the 65 watt version, it's cheaper (if the laptop needs more power it'll take what's needed from the battery) about $36:
http://www.newegg.com/Product/Product.a ... 6834991048
Note that review for this suggests that it may not be able to charge the battery...and has marginal wattage for larger displays. Also at 65W you're trying to operrate at 1/2 of the power that your original adapter had - the more I think about it, the more the 65W model seems like a bad idea - you save nothing if you end up needing a different adapter to charge battery...
Personally I'd get the 95W model, 5.7" x 2.9" x 0.7" is not much larger than the 65W 4.25" x 1.81" x 1.18"