By Portulaca, I think you are referring to P. grandiflora, AKA moss roses. Seeds for these are usually readily available. Do not cover seeds. Needs temps consistently above 70°F to germinate.
Purslane is also Portulaca, P. oleracea. Never noticed seeds for sale but I wasn't looking.
Stores also sell Portulaca umbraticola as ornamental purslane or the other moss roses but I've not seen seeds for those. Not necessarily looking, but would have bought if found. (I only buy seeds in person.)
