Though it has been suggested that methods used for total RNA isolation such as TRIzol (from Thermo Fisher Scientific), may not be completely efficient in preserving smaller RNAs such as miRNAs, many miRNA studies have however been published using TRIzol.
If you would prefer to use a dedicated miRNA isolation kit, variants are available for miRNA isolation from cells, fresh / frozen tissue, and formalin fixed paraffin embedded tissues. miRNA isolation kits supplied by Qiagen and Thermo Fisher Scientific are popular options.
When choosing whether to use a dedicated miRNA isolation kit or a total RNA isolation option such as TRIzol, an important point to consider is whether along with an analysis of miRNA, an analysis of mRNA from the same sample is also desired. Isolation methods such as TRIzol that retain both miRNA and mRNA may in this case be ideal giving the broadest options for future experiments. TRIzol is also a relatively cheap option.
Following miRNA isolation, concentration analysis can be quickly performed using a Nanodrop (from Thermo Fisher Scientific). A Nanodrop takes up very little bench space, requires little training and maintenance and requires no reagents. A 260/280nm ratio of 2 suggests a reasonably pure RNA sample.
For links to molarity and dilution calculators see online science tools