Is there a device that I can purchase to increase the water pressure in my house?

You could turn the water pump up cuz you can adjust the pressure on water pumps

Pressure and volume are not the same when considering water supply. You can have adequate water pressure but not enough water at that pressure.

It can be happening because of debris in your shower head or your valve assembly. Newer, more expensive valves have regulators built into them. It may need looking at.

The dirt in the shower head is your job to take apart and clear out., or exchange the head How much water and pressure come through the shower head pipe with the shower head off? Lots?? Look to the shower head being the problem.

Flush the lines out while you have the head off.

If you have meager water come out the pipe with no shower head on it the pipes or valve is an issue.

The valve would be disassembled to clean it.. The pipes are not so easily cleaned if they are being restrictive.

If none of the above seem to work out, then a booster pump is necessary.

Another thing to check is the size of the pipes. If the runs are long and the pipes are skinny as soon as you start trying to use a lot of water the pressure will drop. Pipes should be 3/4 inch everywhere until they get right up to a fixture. If they are less than 1/2 inch they could easily be responsible for low flow/low pressure even if the static (when you aren't using water) pressure is high.

Seems like you should look inside the shower head and determine if there is a flow restrictor installed there. Remove it if you can. If not, buy one with a larger flow potential.

Here is the biggie: What size pipe did the plumber install? If it isn't 3/4th inch diameter, more than likely you can't help the situation.