How can I find my pet wolves that I told to sit?

If you're not opposed to cheats, you can theoretically teleport just your wolves with a series of three commands:

/scoreboard objectives add mine dummy
/scoreboard players set @e[type=Wolf] mine 1 {Sitting:1}
/tp @e[score_mine_min=1] @p

What this does is sets up a scoreboard to select your wolves, sets it for all wolves that are sitting, and teleports those wolves to you.

Alternatively, you could teleport all wolves with just one command:

/tp @e[type=Wolf] @p

Keep in mind, though, this will only work for wolves in loaded chunks. So you're going to need to be within about 8 chunks of where your wolves were last, and also in the same dimension. But if you know approximately where they are, within several hundred blocks, and are open to cheating, this would work.

If you happened to be on a server, the "all that are sitting" method might not work out too well. For that, assuming you also have access to cheats there, replace Sitting:1 with OwnerUUID="<your UUID here>". You'll need to figure out your UUID and copy it in. But, if you're in single player, checking to see if they are sitting will work just fine.

In newer versions of minecraft you can use NBT in your target selector. Teleporting all of your wolves to you can therefor be done somewhat easily with a command like this:

/tp @e[type=minecraft:wolf,nbt={OwnerUUID:"????????-????-????-????-????????????"}] ~ ~ ~

You have to replace the question marks with your UUID.

You can get your UUID in-game by using /data get entity <target selector for one of your pets> OwnerUUID.
Or you can use this link: https://api.mojang.com/users/profiles/minecraft/<your username>
Or you can take your NBT-data and convert UUIDMost and UUIDLeast into your UUID. I'm not completely sure how to do that and it seems to be somewhat complicated.
Or you can go to the save data of one of your worlds, go into the playerdata-folder (C:\Users\<your name>\AppData\Roaming\.minecraft\saves\<world name>\playerdata) and look at the name of the file inside. There should be one file for every player who has been in that world and the file name is their UUID.

Note: This will only teleport your wolves to you and only those that are currently loaded. If you are too far away they may be sitting in an unloaded chunk, then they won't be affected by the command.

To simply teleport all sitting wolves to you, regardless of who owns them, you can use this command instead:

/tp @e[type=minecraft:wolf,nbt={Sitting:1b}] ~ ~ ~