In the measures subsection of the methods section in the paper you refer to (Gifford et al, 2004), you can see that Gifford et al. (2002) is cited when referring to the AIS questionnaire. However, I cannot seem to find this paper titled "Combining Bupropion SR with acceptance-based behavioral therapy for smoking cessation: Preliminary results from a randomized controlled trial."
Maybe this is what you were referring to when you stated "I cannot find it"?
I did a more general search for "Gifford Avoidance and Inflexibility Scale" and found Gifford & Lillis (2009) in which they cite the original paper you found (Gifford et al., 2004)—hardly useful.
Interestingly, a validation study of the scale was published by different authors (Farris et al., 2015), in which they provide the following citations:
The Avoidance and Inflexibility Scale (AIS; Gifford, 2001; Gifford,
2002; Gifford et al., 2002) is a smoking-specific measure of
experiential avoidance that was specifically developed for
process-based smoking cessation research.
They also provide the following description:
Avoidance and Inflexibility Scale (AIS; Gifford et al., 2004) The AIS
is a 13-item self-report assessment in which respondents to consider
how they respond to difficult thoughts that encourage smoking (e.g.,
“I need a cigarette”, “I wish I could have a cigarette now!”),
different feelings that encourage smoking (e.g., stress, fatigue,
boredom, enjoyment, satisfaction, etc.), and bodily sensations that
encourage smoking (e.g., “physical cravings or withdrawal symptoms”).
Items are rated on a 5-point Likert scale (1 = Not at all to 5 = Very
much), with higher scores reflecting more inflexibility/avoidance in
the presence of difficult smoking-related thoughts, feelings, and
Gifford (2001) is an unpublished manuscript. Gifford (2002) is a doctoral dissertation ("Acceptance based treatment for nicotine dependent smokers: Altering the regulatory functions of smoking related affect, physiological symptoms, and cognition.") which I am also unable to find online. Lastly, Gifford et al. (2002) is the first paper we could not find.
The validation study does provide all 13 items in Table 1 (Farris et al., 2015). Combined with the knowledge that it is a 5-point Likert scale (1= Not at all to 5 = Very much) you do have most of the information needed:
However, personally I would recommend you to contact Gifford and explain your struggles in obtaining access to her research, and kindly request her to place a copy of the unpublished materials on ResearchGate, arxiv, or any similar platform.
Gifford EV. The Avoidance and Inflexibility scale. 2001 Unpublished manuscript.
Gifford EV. Doctoral dissertation. University of Nevada; Reno, NV: 2002. Acceptance based treatment for nicotine dependent smokers: Altering the regulatory functions of smoking related affect, physiological symptoms, and cognition.
Gifford, E. V., Antonuccio, D. O., Kohlenberg, B. S., Hayes, S. C., & Piasecki, M. M. (2002). Combining Bupropion SR with acceptance-based behavioral therapy for smoking cessation: Preliminary results from a randomized controlled trial. In annual meeting of the Association for Advancement of Behavior Therapy, Reno, NV.
Gifford, E. V., Kohlenberg, B. S., Hayes, S. C., Antonuccio, D. O., Piasecki, M. M., Rasmussen-Hall, M. L., & Palm, K. M. (2004). Acceptance-based treatment for smoking cessation. Behavior therapy, 35(4), 689-705.
Gifford, E. V., & Lillis, J. (2009). Avoidance and inflexibility as a common clinical pathway in obesity and smoking treatment. Journal of health psychology, 14(7), 992-996.
Farris, S. G., Zvolensky, M. J., DiBello, A. M., & Schmidt, N. B. (2015). Validation of the Avoidance and Inflexibility Scale (AIS) among treatment-seeking smokers. Psychological assessment, 27(2), 467.