I'm on my 3rd stamp & I've been very happy with Genesee Stamp, 900 Jefferson Rd. (Regional Market complex, 1st building on the left), Rochester, NY. They have same day turnaround.
I paid Ů.90 for my 2 liner that reads
TRACK SERIAL # AT
The font size is such that it fits into blank space on a bill. I then highlight the Serial # and Series. I recently had a WheresGeorge "hit" email from a bank teller who appreciated the size & placement of my stamp. She explained that while she's a friendly Georger, other tellers frown upon "over marked" bills (pulling them for destruction by the Federal Reserve Bank).
While NaviCaching, I recently took a quick dip (don't ask). My stamp bled away from the Georges in my pocket. I went back to Genesee Stamp & they were very gracious about replacing my standard ink & pad with a waterproof product: Aero Brand Indelible, made by Specialty Ink Co. I also contacted Specialty Ink directly & got a free sample of their #141 stamp ink. I've tested both in soapy solution & they're perfect. See me if you need some ink, but you'll need a new pad (ū.95).