I've been using 12g cartridges (Crosman, et. al.) in an Ultraflate w/o problem and w/o spacers for years. I believe the spacer is needed in the Ultraflate2, not the Ultraflate Plus. Here's some confirmation:
Genuine Innovations UltraFlate Plus CO2 Tire Inflator - REI.com
12g CO2 is ~55 cents/cartridge at WallyMart, etc.