Buddhist monks and nuns rely on offerings (donations) from the laity for pretty much everything.
Lay people accumulate merit and generate positive karma by supporting monastics. This support can be given by offering food (vegetarian in this case), new robes*, incidentals required by the monk or nun, or in some circumstances even cash (although cash was historically prohibited and some monastic traditions still maintain this prohibition).
Making offerings to a monastic also allows one to practice generosity and lessen one's own attachment to worldly belongings which, in turn, decreases one's own suffering.
Monks and nuns in the Tibetan traditions generally do accept cash (normally presented in an envelope and sometimes accompanied by a katag) but do not go out and about on alms rounds, likely due to the geography in which the Tibetan traditions of monasticism developed. Instead, sponsors (the laity) would normally visit a monastery or monastic and bring along an offering, or perhaps invite a monk or nun to their home to provide some spiritual service such as reading the scriptures aloud or performing specific ceremonies and make offerings to the monastics in return. This would allow for both the laity and the monastic(s) to accumulate merit and generate positive karma.
You're unlikely to see Karma Tsewang on the streets of Calgary with a begging bowl in hand, but if you'd like to make an offering to him there's a list of items on the amazon.ca wish list linked to with the amazon.ca icon at the bottom of each page.
* Buddhist monastic robes differ from tradition to tradition making it necessary to ensure that one offers the correct style and colour of robes in addition to ensuring proper sizing