I told Boone I'd meet him back at the lucky 38 so that I could partner up with Cass. Now I've gone back to the 38 and he's not there.
Anyone know where he might be?

Try Novac.

Companions are not instantly trasported to 38 or their home place , they take some time to get there , so try later , he'll show up.

It's been a really long time now, I don't think he's coming back.

On top of that. I have now lost Arcade!
Just lost him without telling him to leave. I can't recruit Cass because she says I'm with someone but Arcade is gone.
Any idea how to get a lost companion?

It hasn't happened to me yet, but it is quite common. A tip flying round is to go to vault 23/22 (can't remember exactly - the one for "there stands the grass" quest) and take the elevator to the bottom floor. Rumour has it your missing companion will be waiting for you there. However, this goes for people currently your companion - in your case Arcade? So Lord knows where Boone's got to. Are you on hardcoe? Is it possible for him to die while traveling back home/Lucky 38?

Wait 24 hours 3-5 days in a row, it'll say he has returned to Novac/The fort for arcade. Hope I helped!!!

Your compaions will WALK (not run) to wherever you tell them to go. You can actually follow them home if you wish, but be prepared for a long walk. If you tell them to walk home from an area (like the Legions camp) it is unclear as to whether they can get acorss the water or not. Also, if on hardcoe, it is unclear as to whether or not they can die on their walk home (being followed in another thread). Just wait in the presidential suite for a couple days and he should pop-in. Check all the rooms as I know that ED-E seems to appear randomly in different rooms when sent to the Lucky 28, but when sent to PRIMM he is always in the house where you found him.

Same exact thing happened to me, took about 7 in-game days for him to show up, so it may take a while.

Companions can die on the walk back; hopefully that's not what happened to Boone, but it's possible.

If you're on PC, look up the method for warping to a given NPC (I don't know what it is offhand) and find out where they are that way.

RE Boone.All those Mysterious meat's you find? Yea...sorry buddy.