Walmart has the 7 Hanes Men's Fresh IQ Tagless Tank Tops (White) for $13 with free in-store pickup or $5.99 shipping to home. List price: $17. They're available in sizes S to XL.
Amazon has the Hanes Women's Signature Breathe Cotton Hipster 6-Pack for $10 with free shipping for Prime members or on $25+. Has average Amazon Reviews.
Amazon has the Calvin Klein Underwear Men's Cotton Classics 4 Pack Boxer Briefs (Black) for $25 with free shipping on $25+ or with Prime. List price: $43. Has good Amazon Reviews.
Amazon has the 5-Pack Gildan Men’s V-Neck T-Shirts (Assorted Colors) for $9.00 with free shipping on $25+ or with Prime. List price: $15. Available in select sizes S to XL.
Has good Amazon Reviews.