What's the year, make and model of the vehicle?

It's a 4T65-E mated to a Buick Series III 3800, 2006/2007 manufacture.

Engine model is L26, I do believe, and the transmission line should be M15.

Confirmed via the RPO codes; M15 transmission on an L26 3800, running either 3.05 or 3.29 gears, but from the way it pulls and how most models in that range were built, I do believe it's the 3.29 gears.

*stares* I just asked the year make and model of the vehicle.. not all the part number, you goof. Either way, if you want I can Check Ebay and the various junkers down here in tucson, there's "you-pull-em" car parts places all over town

The model numbers of the transmission and engine are actually more accurate than the make and model of the car; when parts dealers look up things, they don't necessarily know what they're doing, just following a script. Those transmissions were actually widely used across platforms, so you could - theoretically - take the trans off a Bonny, Lacrosse, Impala, or possibly even the HD version off a Grand Prix and bolt it right up to the engine. If they go looking by just car year and model, they might not find a transmission even though they have a compatible one in stock, because the tag might be for, say, a Bonneville.

I don't know the difference between the HD and "regular" versions of the 4T65-E, but I want to say the HD version just has stronger gears to handle the added power coming down the flywheel.

If you could find one in Tucson from a salvage yard or a nearby dealer, that'd actually be great; I'll be doing my own search around here to see if I can find anything. Preferably, I would actually like to have a second transmission, just so I can take the old one apart and put it back together again for practice, so the next time one of my transmissions goes out, I can rebuild it myself.

I'm having trouble, btw, identifying precisely which models got the M15 4T65-E with 3.29 gears, but a dealership ought to be able to tell you. I might have to do the same thing myself so I can pester salvage yards for a compatible trans.

"What about for this model? And this model? And this model?"
"WHY DO YOU KEEP ASKING?!"
"It was all the same transmission."
"(mind BLOWN)"

Also, I spotted a couple on ebay already, but I'm not sure of the compatibility just yet... I'll need to ask them, but I'll also need to see if I'll even have the money to get them first.

Oh, but it's a 2007 Buick Lucerne CXL; 3.8L V6 (the Buick and Northstar motors used different transmissions; be careful not to get stuck looking for the 4T80-E... that trans only fits the Northstar motor... I think it might even have been made just for Cadillac, actually).

I worked for GM when they rolled out the Lucerne, originally they all came with caddy motors

Glad they came to their senses and put the GOOD motor in, then.

Also, a Buick with a Caddy motor is just a travesty, especially when it's the POS Northstar.