I know it's too late to help you but local fish stores may sell or give you some bags.
Also, while you are at the swap meet, see if anyone is selling fish bags. They are usually available at our auctions and meetings.
I don't know of any fish club that permits ziploc or food storage bags as an appropriate container for meets, auctions or club events. Most will allow clear plastic containers or buckets with lids, as long as they don't leak.
Eheim Pimpette #270
Eheim Pro II 2028; Classics Series -2215, 2217, 2260, 2262; Pro 2229 Wet/Dry