Where is the orifice tube on a 98 Buick LeSabre located?

Not exactly sure but using info below you can find it.

Remember the flow for the a/c system is :
1. From the compressor a high pressure (HP) gas goes to the condenser in front of the radiator.
2. From the condenser a HP liquid goes to an expansion device (orifice tube or expansion valve)
3. From expansion device the low pressure gas enters the firewall to the evaporator.
4. From evaporator the low pressure liquid/gas mixture goes through the dryer back to the compressor suction.

There will be about a 1" nut on the low pressure line between the firewall and the compressor. Remove nut and on the male side use a pair of kneedle nose pliers and remove orifice tube. Note orientation of tube.

Replace tube, reassemble, evacuate air from system, recharge with freon.

On the a/c system...if you have to ask, take it to the shop. This is not a backyard mechanic job.

It's in the high side line coming from the condenser. It's the line just below the low pressure line.
on a Buick LeSabre you take out the battery to gain access to it.
Take out the battery and disconnect the low side line.
Now you can remove the high side line from the condenser. The orifice tube is inside the line.
Remove the opposite end from the evaporator at the firewall area to give you room to remove the line from the condenser.