hi, you can visit thermo fisher company and look Oligos, Primers, Probes & Nucleotides section. if you want make custume primer sequence they will provided.
I would agree that thermofisher is useful for primers (although it's been years since I've needed to design a primer). To look for/confirm miRNA sequences, you should definitely use mirbase. They have a downloads section that allows you to download all miRNA sequences across all species that mirbase has on file.