Walmart across the street from Costco in Warrenton most likely has booster shots if Costco doesn’t. Walmart’s website is set up for booster appointments (e.g. if you got J&J, they’ll let you sign up for a Moderna booster). I got my Moderna booster at a Walmart in Portland. All the Walmarts carry different vaccines – some don’t carry them all. Most pharmacies are taking walk-ins too I think – Walmart is.
I think it’s fine to get a Pfizer booster if you had two Pfizer shots originally. Getting Moderna might have been marginally better but maybe not much. Wouldn’t have gotten a J&J boost most likely – but I’d like to see more efficacy data on the mix-and-match and I don’t think there is really any – just studies of increased immune response.