You can order oligos from a commercial provider (Sigma, IDT, etc.) and for each one of the nucleotide positions, you enter the letter "N" which means they will use A, C, G and T to synthesize the oligo at every position. This will theoretically generate all possible permutations of the oligo sequence which will then 'randomly' prime cDNA synthesis, whether you design the oligo to be six bases long or nine. We have used random hexamers with good success.

For cDNA synthesis, you would typically use a kit that includes random primers (usually hexamers). It would be wildly difficult and time consuming to do it by ordering a tube of reverse transcriptase enzyme, and ordering and designing primers separately.

Kits are quite expensive and if you do get the oligo NNNNNN or even n9 made you do get a huge amount of primer for very little cost Whether to use a kit or make your own depends on how many samples and how available your samples are. \kits are great for very valuable and rare samples but if you have plenty of material it can be worth setting up your own reagents

Dear Paul, How can I do it!

You can order oligos from a commercial provider (Sigma, IDT, etc.) and for each one of the nucleotide positions, you enter the letter "N" which means they will use A, C, G and T to synthesize the oligo at every position. This will theoretically generate all possible permutations of the oligo sequence which will then 'randomly' prime cDNA synthesis, whether you design the oligo to be six bases long or nine. We have used random hexamers with good success.

How about using OligoDT primer (16 to 20 T will do it)? This will select mRNAs with polyadenylated tail from the total RNA isolated.