Penny says on the show that it is from Forever 21, I haven't found it online yet though.
Penny’s blue striped sweater on The Big Bang Theory:
Striped Sweater by Derek Lam 10 Crosby at The Outnet, $110 (was $295)
Penny’s black floral top on The Big Bang Theory:
Villeaw Kensington Floral Tee by Ted Baker at Nordstrom Rack, $39.97 (was $79)
Penny’s rose print sweater on The Big Bang Theory:
‘Melanie’ Floral Sweater by Equipment at Nordstrom Rack, $114.97 (was $318)