Bottles aren't rated as 'HP' or 'LP' as such. You can simply fill them to a specified pressure which is then used to power your marker. The reg attatched to the bottle decides whether 'HP' (approx 800psi) or 'LP' (approx 450psi) is actually constantly output to your marker.
You can get a bottle which will fill to 3000psi (which gets regulated down to high or low press by the reg) and you'll get so many shots from that.
If you have a 4500psi bottle the same thing happens, but you'll get more shots off for a full fill of the bottle.