What kind of fish you catch is determined by map data. If you can't catch certain type of fish, despite trying many times with correct bait, you need to find a different spot. Fishing page contains information of baits known to work with each fish.
According to current data on the Fishing page, best place to catch Plaice will be a river or shallow lake near a swamp that provides Bloated Leech. This way you can have practically infinite bait, limited only by your Hitpoints and time it takes for leeches to bite your character. Be careful when letting Leeches attach or putting them on, as you can run out of SHP (the first of three numbers on your lifebar) and get knocked out, gaining Concussion damage (also debuffing your Abilities) and Leech Burns if there are hungry, unbloated leeches still attached to your character. You can remove unbloated leeches as long as your character is not knocked out. Leeches attach to your empty equipment slots, meaning you can get naked to quickly acquire leeches or fully equip yourself to prevent them from attaching. Notably, bloated leeches can be used to block fresh ones as they won't become unbloated (and hungry) again unless removed.
I'm especially proud of the first paragraph, and dare to suggest (that after some peer-review) it should be copied into every fish page.