The SMC SY3000-23-4 screw is a M2x21mm long "round head combination screw", according to
http://content2.smcetech.com/pdf/SY3.5.7.9000.pdf, page 29 of the PDF. At a guess, McMaster-Carr or Fastenal could get you some M2x0.4 x 20mm in either pan or cheesehead (I'd go with cheesehead first for the smaller diameter head).
Fastenal M2x0.4 x 20mm Pan Head - 0173301 $0.0793 each
Fastenal M2x0.4 x 20mm Cheese Head - 11541976 $110.80 per 1000
McMaster M2x0.4 x 20mm Pan Head - 92005A039 $5.96 per 100
McMaster M2x0.4 x 20mm Cheese Head - 91800A022 $8.33 per 100
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For the gasket,
http://www.smcpneumatics.com/SY3000-11-25.html lists 20 in their warehouse #2 at $1.10 each. I'm not finding a direct equivalent, but at that price, it's probably not worth searching very hard.
Or you could go to your local SMC distributor which is probably:
Allied Electronics - Suffolk County Area
2150 Joshua's Path, Suite 14
Hauppauge, NY 11788
Phone: (800) 433-5700
Fax: (631) 234-5175
http://www.alliedelec.com
And see if they have any that "fell on the floor" (ie - could be donated). Not being familiar with your area, I'd try
http://www.smcusa.com/top-navigation/contact-us.aspx and pull down your state.