You can get LEDs that already have the leads soldered to them on ebay.

You will have to add the resistor, unless they are already installed (ie: 6v or 12v LED)

what game are you trying to connect from? if 90's bally/wms and sterns up to say a few years ago you can use a matrix adaptor from Comet Pinball and then easily adapt your wiring to a particular feature then put the correct rated resistor on to the longer leg of the led and when a feature is lit for a GI socket or insert your modded led when turned on.

http://www.cometpinball.com/category-s/1871.htm

For 6.3 volts AC, you can use a 220 ohm @ 1/4 watt resistor in series with the LED.

Several years ago I found a website (resistor wizard) that shows what you need. Click on led calculator if you are just using one.

IMG_2383 (resized).PNG

If you are connecting a standard SMD LED to AC GI, I would think you would need some kind of rectification to push it to DC of the proper bias. Here's a quick and dirty result for a supplier of small LED's

https://www.superbrightleds.com/cat/surface-mount-smd/filter/LED_Type,2835_SMD,50,5904:

However, if you have any old electronics with displays lying around, you could potentially scavenge the LED's out of the backlight. They are typically 1x4 mm or so. Depends on how much you want to save on shipping Although, some LEDs used here have 2 chips in series (bright!!!) and run around 6V instead of 3V.