Hi there. Maybe you can try to set up a difussion or permeation source, which is relatively easy to build in your lab. I have been using such systems to prepare gas standards for VOCs analysis for several years. Teflon tubing or capillary tubes may give you the 20 mg/day you need. Also, using a dynamic dilution technque, you might be able to generate standards in different concentrations.

I recommend you to check this paper out to get further information:

DOI 10.1007/s00216-012-6202-5

Hello. You might try to consider a specialize company that delivers special gases, like LINDE GAS

A permeation source would be the best candidate. It can be built at a reasonable low cost. It can be easily modulated in terms of permeation rate by just changing the length, thickness and chemical nature of permeation material. Also changes in temperature may provide enough modulation. The only limitation is due to the fact that such system need a long time to stabilize (usually several hours), thus it must be maintained in a controlled temperature  bath, As GEH suggests, an addition dynamic dilutor may provide further flexibility in changing mass and concentration output.

Thanks very much guys for your valuable thoughts... I was aiming for higher mass loss say in 1000 micrograms/day at very stable emission rate. After trying different things I saw reed stick in a shopping mall and thought of using it. I put the reed stick in open 50 ml volumetric flask with 30 ml Toluene and it gave very high mass loss. Yes it takes very less time to give stable emission.