Want to put a passenger seat on my big dog bulldog

Very expensive

https://www.google.com/search?q=2000+Big+Dog+Bulldog+repair+manual

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Replace the spring with a heavier duty should do the job.
Have you tried adjustments in the spring load to stiffen it up.
The rear shock on your Yamaha V-Star 1100 motorcycle assures that the rear wheel is always in contact with the ground and makes your ride much more comfortable by absorbing road vibration and shock while you ride.

But your shock needs to be adjusted for different riding styles, so depending on whether you ride fast or cruise slowly and how quickly you take turns, you may want to adjust the shock to get the most out of your ride.

1

Remove the passenger seat by loosening and removing the rear passenger seat mounting bolt with a socket wrench.
With the passenger seat removed, you should see two larger rider's seat mounting bolts.
Remove these bolts and pull the rider's seat off of the bike to expose the internals of the motorcycle. 2

Detach the quick-release clips of the ignitor unit found just beneath the seat using a screwdriver. Pull the ignitor unit up and to the right of the bike to move it out of your way.

Remove the mudguard quick-release clips in the same fashion, then pull the mudguard out of the bike to expose the rear shock adjuster. 3

Look for the small numbers on the rear shock adjuster.
On most 2002 V-Stars you should see the numbers 1 to 9 printed on the shock adjuster.

These numbers give you an idea of the current setting of the shock.
The higher the number, the stiffer the adjustment. 4

Loosen the shock by turning the shock adjuster counterclockwise to a smaller number using a spanner wrench.

This makes the shock softer and more forgiving when riding over bumps and uneven pavement, but will feel slower in turns and accelerations.

Tighten the shock by turning it clockwise to a higher number.
This makes the shock stiffer, increasing road response in turns and giving you a better feel of road conditions at the expense of comfort. 5

Replace the mudguard, ignitor unit, rider's seat and mounting bolts, and the passenger seat and mounting bolts.

Take your bike for a ride to feel the difference a turn of the shock adjuster makes.

Lift the rear of the passenger seat's cushion to find the screw that secures the seat to the rear fender. Use a 5-mm Allen wrench to remove the screw.

Lift the rear of the passenger seat slightly and pull it away from the rider's seat.

Use the 5-mm Allen wrench to remove the screws from the rear of the rider's seat.

Lift the rear of the rider's seat upward slightly. Pull the seat back until the tabs on the front of the seat separate from the motorcycle's frame.

If you put your ignition key in the lock alongdide the rear seat and turn anti clockwise, the rear seat lifts from there and when removed the front seat will lift up.
Hope that helps.

First thing remove the screw holding the passenger seat pillion. Next remove the 2 mounting screws that hold the riders seat. Once the screws are removed you can pull the seat toward the back of the bike to remove it.

I’m happy to help further over the phone at https://www.6ya.com/expert/al_e83b1941f9c0f302

First, remove the passenger seat; one bolt behind it anchors it to the fender; then lift the back end and push against the rider seat to disengage the keyhole mount to the front bolt. It pops up when you do this.

The bolt that engages the passenger seat bracket also anchors the rider seat. Remove it and lift the seat off.

An electronic (expensive) module clips in above the battery. Remove it and handle it carefully. Now you can get the battery.

I learned this by experience on a bike I had never seen before, so I checked the Clymer manual. This is what it says too. Worth the money!

The battery is under the seat on the right side of the bike. You must first remove the seat by removing the screw on the top of the rear fender that holds the seat in place...you can't miss the screw, it's a big chrome philips head screw. Once the screw is removed, lift the seat up slightly and gently pull towards the rear of the bike; this will release the front of the seat from it's retaining tab. Now that the seat is off, look at the right side of the bike, the chrome box there is the battery compartment. I believe the bolts connecting the battery are 10mm...you can undo them with a wrench and pull the battery out.

the tab is part of the unit that holds the hold-on-strap to the bike. the two bolts used go through the hold-on-strap unit then into two holes at the rear of the front seat.
as for where you can get them..... dealer or scrap yard.
hope this helps.