IT seems to vary by area of the world, but I find that monitors with DVI-D input are far more common than ones with HDMI. The two signalling systems are basically the same, but the connectors are different. You can get inexpensive cables with HDMI on one end and DVI on the other.
Once you find one with either DVI or HDMI, pretty much any monitor will do. I get mine from a local thrift/second-hand shop. I've gotten a 17" 1280x1024 flat panel monitor for as little as $8.50. It is very rare these days that I pay more than $20 for a 19" panel. You can also find monitors on Freecycle (if you have that where you live) and some people have gotten monitors from companies that are replacing desktop systems. Companies are *eager* to have someone cart such equipment away because that saves them disposal and recycling costs. You may have friends or relatives that have monitors that are not in use that they might give you, or let you use as a long-term loan.
Really...finding a suitable monitor for a Pi is a trade off between time and money.