I get mine on-line, but, a good body repair shop may handle it or another brand that is similar (there are many). POR-15, and its familiars, are usually termed "rust converters" or other closely worded references. Rustoleum is designed primarily to "cover and stabilize rust", while the others are touted to actually "convert" the rust. I've found that they do a much better job, but, they are more pricey than the commonly found Rustoleum products.
My 2 cents, your mileage may vary...
2 Cats; J-Lo and Ragamuffin :R
2014 Thor Tuscany 40RX DP
2011 Ram 2500 Longhorn CTD HO
2011 Keystone Cougar 318SAB (now gone)
2008 FunFinder X 210WBS (Sadly gone)