The pay satellite services transmit at a higher power level so that you can use a smaller dish.
33" isn't too small, I'm using a 30" dish. Some satellites are stronger than others though, and they aren't all uniform across the country either. Did you have a specific satellite you were thinking of trying?
You can replace the LNB on an 18" dish, but you might have to build your own mount for it. There's only one linear LNB made that slips into the end of an 18" DirecTV dish tube. But this won't work for all transponders on all satellites in all areas. In other words, it's probably for people who already know what they're doing.
The LNB receives both horizontal and vertical polarization, selected by the supply voltage. Alternating transponders on a given satellite are opposite polarities, and corresponding frequencies on adjacent satellites are (supposedly) opposite polarities, in order to reduce interference. (This is exactly the same with pay satellites, except the polarization is left-hand and right-hand circular.)