try an online search -- there are a lot of sources for prefab counters, or you can probably get a piece of something at a discount from a stone yard or fabricator.

Are you looking at a stone or laminate?

There are several options. What you pick will depend in part on your budget and what is suitable for your home. I live in a neighborhood of $200,000 to $350,000 homes (in the Midwest - so ordinary, but nice middle class homes) and most homes in my neighborhood do not have stone bathroom counters - though some are remodeling kitchens with granite or quartz.

Some stone remnants are very reasonably priced, but they can be a little more to install due to the care that needs to be taken and the weight. The fabricator will also need to cut the sink hole and faucet holes - a bit harder on a stone than a laminate, though not really a problem for an experienced fabricator / installer. I have seen quartz for as low as $35 a sq. ft., but that is uninstalled.

Laminate can vary a great deal in price - some get nearly as expensive as a stone. The least expensive laminate would be an in-stock laminate at a home center store. This 6 ft in-stock is $30 on sale.

https://www.menards.com/main/kitchen/countertops-laminate/view-all-countertops-laminate/customcraft-countertops-trade-6-ft-argento-romano-modern-edge-laminate-countertop/p-1455974621236.htm

You would still need to put the end caps on (about $9 each), have the sink hole cut and faucet holes cut as part of the installation - so some labor cost involved. Also, for both a stone and a laminate you need to also buy a sink. Stones and laminates are the most durable choices.

Another possibility is a cultured marble top. These include the sink and are easy to keep clean because there is no caulk line between the sink and counter as it is all one piece. They do scratch easily. This one is listed for $462.

Double Bowl Cultured Marble Vanity Top 73", Solid White · More Info

One good reason to have a fabricator make a countertop out of remnants for you is that not many walls are square. What ends up happening is the counter will not fit correctly, leaving an unslightly gap at one end or the other. The sink(s) also may not be centered on the sink base cabinets either.